AWESOME progress.
And this is where I start to enjoy where you go down a different path of "what might have been. " .
I really like that you are going with wheel-opening mods. A lot of thoughts happened when I was doing this, but the mantra was always " No body mods".
Really looking forward to progress from this point on.
Amazing work.
More progress. Still using 18 ga on most of the inner fenders and firewall.
I slanted the firewall towards the outside to allow for some sort of hood latch mount. Probably just simple SS hood pin style latches (not much to fail with them). I left a large opening where I will route the wiper transmission cable tube though a removable steel plat with a rubber grommet. This is the wiper kit I am using (I also used this on 46 Packard 15 yrs ago). https://www.specialtypowerwindows.com/power-windshield-wiper-kits
I did use 16 ga for about 6" above the shock tower. The black smudge marks are where traces of seam sealer caught fire from welding.
Right side will house the Miata fuse box assembly. I think I will drill a couple of 3/8" drain holes in the recessed area.
Cowl drain from Miata. I sized the inner fender to clear 195/65R15 tires at full lock and shock fully collapsed.
I have to replace the top of the inner fender braces where the fenders weld since they are crusty/ rusty and then i can move to the rear of the car to finish the splices.
Lots of progress recently.
Volvo ground clearance of 4" at crossmember with Miata 195R50-15 tire at approximate ride height. I will be using 195R65-15 which will raise the ground clearance ~ 1.2" to 5.2" which is streetable. I am using 3/8" all thread rod set to mid-stroke of the shock travel to set the level
I made my first cuts on the rear wheel wells so I could get my rear tires on.
The car had the original red paint and one repaint in bronze. I stripped the top layer with 36 grit on a 7" wheel. Then used Stripping Wheel for Angle Grinder 4-1/2" x 7/8" from Amazon, they really worked fast, strips the paint with no metal loss.
Tight areas were stripped with a coarse 4 1/2" wire wheel.
Due to a large amount of bare metal areas with surface rust I will lightly blast the whole body.
I will construct the perimeter of the new wheel openings and the exterior rockers and fender bottom and rear quater panel bottoms with 18 ga steel tubing (maybe 1/2" EMT conduit) then blend to the body with multiple 20 ga panels made with a shot bag and hammer and a little English wheel work . I have found if you fill the tube with sand and crimp the ends you can bend without distorting the radius of the tubing and not cause crimps
Miata trunk floor waiting to be spliced to the Volvo.
The Miata dash rough fit to the Volvo (new side brackets are welded in). The ragged edge of the dash and the ends will be made from fiberglass and resin and polyester body filler. I will close the Air Bag hole with multiple layers of fiberglass and resin. I plan on slicking the dash out with polyester body filler and sand smooth to make the dash look like steel.
My friend with a lathe made a steering column extension that I will weld in place (a lot safer to me than a Borgeson type joint/ coupler with the mickey mouse 1/4" set screws). I cut the shaft and inserted the extension which is machined to slip through the Delrin bushing in the firewall rubber seal.
Trying to figure out where your dash defrost vents went?
Also have you decided to leave the fuel tank in the Miata location or are you going to move it? My original plan to extend the Miata at the rear was to harvest a trunk section out of another Miata, flip it around and weld it to the one in the car giving me a longer spare wheel-well. But not enough room for a fuel tank anywhere. and no room for muffler.
The wiper kit you bought is the exact one I bought. I run it off the Miata stalk control but had to reverse the polarity of the speed signal from the stalk. That may or may not have been a Canadian car thing. I also had to modify the drive wiper cable drive box as it was poorly designed and flimsy.
In reply to NOHOME :
Re: Trying to figure out where your dash defrost vents went?
Maybe the Miata NB is slightly different shape than NA ( I think it is different because I had to trim the width about 1/2" at the A pillars). Also since my floor is raised from yours to lower the car and the Volvo cowl/ firewall is sloped forward the windshield opening intrudes on the dash. I also rolled the dash so that the front is a little lower on the front (or my dash would have been above the windshield opening).
Is the wiper kit you bought the exact same brand from Specialty Power Windows ? There are copies of that design that are flimsy (I know because I bought a cheap copy 15 years ago that wouldn't work properly and changed to the Specialty Power Windows brand).
I am going to use the Miata tank and parts of the filler neck (the filler neck is too tall for the Volvo).
I am using a Bayonet Filler Neck 2.28 O.D. (for older GM style vented caps). I will use JB Weld to adapt the filler to the neck. My 1964 P1800S has the gas filler on the left rear corner next to the fin. To gain vertical height I am relocating the fuel filler compartment and door forward to the top edge of the trunk lid like the Miata filler is to elevate the neck about 2". This is necessary because the Miata tank sits much higher than the Volvo trunk mounted tank.
I am eliminating the fuel vapor recovery system (like the Volvo was) and will vent to atmosphere using the Miata filter on the end of the vent tube to keep critters and dust out.
I did some of the paint stripping in tight places and around the fins with a 4 1/2" wire wheel. This is what happens when your arms get tired and you relax. 
Last time this happened was 45 years ago when I was 30 yrs old running a heavy 7" grinder with a cup style wire wheel - ouch, it still hurts to think about it.
I lucked out on the injury.
There is not a time I grab a grinder tool that I don't put one of these images in my mind. Makes me hang on just that much tighter.
Project is coming along great.
Pete
These are close to the stance I'm trying to achieve.
Because of the Miata wider track the fenders will have to be flared though Something like this.
Hopefully not to this extreme.
I like the second pic!
Third pic for me. Stance and wheel/tire fitment is just nailed on it.
What part of the deep south are you in, seginc?
In reply to Dusterbd13-michael :
Birmingham, AL
Getting ready for 2 coats of epoxy primer today - some nice metal under all the ugliness. I don't normally paint on the lift side of the shop but I needed to paint some underneath
I use https://www.summitracing.com/parts/sum-swsp312g-12 which like all Summit Racing Paints is made by Kirker Automotive. You can buy Kirker branded products or the identical Summit branded product usually cheaper from Summit Racing. I have been using their full line of products for more than 15 years with no failures of any kind. Top coats whether single stage or base coat only come in a limited number of factory mixed colors. I save at least 50 % on my total paint costs.
The roof and a few other areas had significant surface rust damage where the old paint had lost adhesion. Some body filler knifed in and then some high build polyester primer will take care of that.
Wow, this is looking beautiful. Nice work
I tried the low stance and it was too much for me.
Funny I should post this pic today as I am waiting in the shop for someone to come adopt the wheels shown.
Polyester primer over epoxy primer after thoroughly cleaning with Prepsall , polyester primer only on exterior metal to make my first guide coat sanding with 80 grit.
Moved the gas filler compartment from rear edge of rear fender forward to accommodate the Miata fuel tank position.
Miata OEM fuel tank location eats up Volvo back seat space. I will not have any evaporative emissions equipment (some parts and hoses still showing in the pic) and I will plug the rollover valve. I am modifying the Miata fuel filler neck for a 60s/ 70s GM style vented cap. I will leave the plastic check valve in the Miata filler neck to minimize the slosh back of fuel into the neck.
Since I turn 75 soon I think I will paint under the hood with black epoxy instead of body color to limit the number of times the engine and trans have to go in and out.
Notice the major crossmember cut to be able to roll the rack and pinion enough to clear the Miata OEM A/C compressor. I will smooth out the surgery on the crossmember with a little body filler then epoxy prime over the filler. I will use brushable seam sealer just like the factory on most of the other weld seams.
Trunk floor needs done. the mild fender flares (about 1.5") will be a challenge. I hope that my theoretical ride height calculations are right so the wheel openings won't be too high (ugh) or too low (ouch) but just right 
.
I metal worked the tail panel wrinkles out decent enough for a body filler skim coat.
Good progress. Hope I am still doing this stuff when I am 75.
A good way to finish the trunk would be to find another donor miata and harvest the last foot of the trunk and frame section and flip it around to match the end of what you already have.
Pete
Front clip from the 1970 P1800 on the 1964 body. Straps to pull fenders back tight as possible. Pop rivet temporarily while setting hood gap.
Coming soon will be the fender flares (as little as possible) to accommodate the wider Miata wheel track and lowering the car.
I am liking the bodywork aspect of the project. I can't wait to see how the flares and stance come out in the end. Long past questioning your sanity or your skills and just enjoying the show.
Pete
Work begins on my very ragged rocker panels.
I decided to make the rockers a multipiece weldment instead of a 1 piece formed. It is very hard to find the exact angles to bend and then bend the angles accurately. Also there is a slight crown outward about in the middle of the door . To work around this I bent a 90 deg angle piece from 20 ga for the bottom of the rocker. I then clamped .5 in dia 16 ga steel tube and welded from the backside of the tube so I wouldn't have to grind the welds. This weldment was clamped to the inside vertical rocker and welded. 
Now I need to create the outside vertical rocker skin from 20 ga using a slip roll to create the rocker curve profile.
Now I can weld this to the bottom while piecing in to the top metal that is good. Because of the 1/2" round tube the angle is automatic because you just weld to to the tangent of the tube circle.
With both rockers finished I can move on to the complex curves of the rear quarters behind the rear tires.
Rear rear inner and outer quarter repair. Not pretty but it will finish out OK.
Front wheel wells 1st try at ride height and fully compressed. I am using 3/8" steel fuel line for the finished edge on the wheel well.
All of the pics below are the first try at shaping the wheel well openings. The tubing mimics the stock Volvo shape except stretched upward to allow for the lowered stance.
First pic at ride height next is compressed.
The pics below show the suspension fully compressed.
This reveals that I am going to have to move the wheel opening behind the back tire some more than the stock Volvo shape to clear the back edge of the tire where the body gets more narrow. Also I think if I move the rear of the wheel opening back a little I can bring the flare inward because I don't like that much flare.
Needless to say, this will take several more iterations to get the flares to look acceptable to me !
I may reshape the rear of the opening like the Aston Martin DB4 below.
Wow, your work on the rear is pretty masterful. What a build! following along
Rear fender wheel wells second iteration, 195/65 R15 tires are at expected ride height according to my calculations. My intention is to replace the Miata rims with something later.
I have about 1/2" clearance for the fully compressed suspension.
The upper 2/3 of the inner fenders are tacked to the 3/8 steel tubing that will be the finished fender lip. Still lots of tweaking to get the lower portions contoured properly,
That looks goooooood!
Can't tell from the angle of the picture, will you have enough clearance for wheel travel with that contour on the inner fenders?
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