SWMBO's 66 Mustang. Trying to build a driver

Since the wife and I got married, she has been wanting to get into cars and learn more. I had the firechicken I was turning into a drag car and it was a little too much for her. She needed something simpler and cooler to learn on. A few days later I was working on a 60s Dodge, and I realized, Old cars are SOOO simple. So I sold the firechicken to a friend and set out looking for and old car for us. Originally I wanted an old C10 but they had gone crazy. I used to have a super minty 69 Firebird I bought from the original owner. I wanted a 67-68 Firebird, but they were thin on the ground and expensive. All the GM stuff I liked was expensive. My mustang friends tried to convince me of an early mustang, I checked some of the suppliers and the parts are as readily available as the popular GM stuff and as cheap or cheaper. Parts availability combined with the ability to find a modern driveline that recently ran and dropped right in (explorer 5.0 and mustang T5 Im looking at you) sealed the deal.

A few weeks later we purchased a running and driving 66 Mustang. Its an auto and a 6 cyl but seemed pretty solid compared to all the ones we looked at.

It did run and drive, but it had some whisky dents, and the drip rails are horrendous.

It didnt seem to have much structural rot (or atleast I thought) compared to all the other ones I looked at.

Drip rails

Door

Quarter

Cowl

First step was to pull the interior because I knew the cowl was allowing water in and figured the pans had some damage.

This things got a working power driver seat

I pulled the carpet up and found this. Wet. My confidence was dwindling

Front of the pans that are always wrecked looks pretty good so thats nice.

Weirdly, the back of the pans are destroyed

I'm following this, bud. I love some 65/66 Mustangs. Luckily- as you know, all these parts are readily available, and you just have to be handy- not good per se, but handy with a mig welder. 

This is were I stopped. I started looking into what it would take for the rust repair, and I got overwhelmed. The plan was to build a shop in January but with the rising cost of metal estimated cost pretty much doubled. I didnt have a big space to work on this, and to properly fix the cowl I needed to pull the winDshield, and I didnt have a place to keep it covered. To do a complete pan I needed a place where I could jig this thing up level and leave it. The garage on the old house is small and wouldnt allow space to work. So I was stuck until I got a shop. We decided we would sell it and find something more solid. We looked at late 2nd gen firebirds and almost bought a 56 Chevy wagon, but it had title problems. Couldnt find anything we liked.

I told her I might as well do what I could, and if I get stuck Ill figure it out, farm it out, or give up, but Id atleast try.

So the other day I went out and started on the least damaged side of the pans. I grinded off and glass blasted most of the scale to get an idea of what I had.

I found out that the pans have been patched before, and the driver side seat riser has been lowered. In the picture you can see on the tunnel where to old flange use to mate to the tunnel. Its been lowered about an inch, which im happy with because I barely fit in this car. Im not very happy with how crappy it is. I also drilled out the spot welds and pulled out the old lower flanges of the seat riser. I ordered a patch for the rear part of the pan. I think I may have enough solid metal to not have to pull the seat rise, but I will reweld it and make it more solid.

Ideally I would do the whole pan, but im not good enough at this to feel comfortable with that. Ive done some welding but not really in a bodywork/automotive setting. Dont roast me too bad, Im taking the roadkill approach and "Dont get it right, get it running." Id like it to be safe but Im not interested in cutting out every little peice of rust. 

I cut out the worst of it in the general shape of my patch panel, and hit the spotwelds on the old flanges and pulled them off, and had to stop so i hit it with some primer I had laying around to keep it from rusting back up.

The flange I have left behind the seat riser has some pitting but is still pretty solid, so hopefully I can get my patch lap welded in. 

When Im done I want to coat the entire inside and underside of the floors so I dont have to worry about it coming back. Im a little unsure about rust encapsulator vs rust converter. What should I put on the metal where it is lapped over to prevent a rust sandwich?

"What shoudl I put on the metal where it is lapped over to prevent a rust sandwhich?"

Grind to clean metal (or blast) and use weld thru primer where your panels overlap. When you're finished welding shoot it with epoxy primer and then seam seal it.