Rear view
Rear view
A-pillar bar landing pads coming together. Take a close look and you'll know why I call this part 'Notch-0'. 
Those front rocker box / landing pads were just too plain. Time to add some style points!
Next was making up a jig to simplify getting the A-pilllar bars in the same place, side to side, as well as making it easily repeatable when doing the 'in/out' to check fit.
The angle I needed was too steep and deep for my notcher so it took a bit of band sawing. Here are some of the remnants of sneaking up on the final miter - which, of course, needed to ever so slightly contact part of the bend. It's fun to drive around corners but mitering in a bit of a curve? Not so much.
Done! It took a fair bit of hand filing to get it to this point.
All the work was worth it. It fits perfectly, with no gaps anywhere. With all the dimensions now known, the other side will go MUCH more quickly.
I plan gussets from the A-pillar bar to the halo and I'm thinking a large-ish, dimple die'd taco plate sort of gusset in the A-pillar bend.
In reply to tygaboy :
Looks good. Neat project for sure.
The only thing you may want to verify is with whatever sanctioning body you may run with and whether or not they allow dimple die stamped main hoop mounting plates.
To be fair the stuff I build is all NHRA & SCTA and such and isn't for SCCA/NASA/Etc, but I know they wouldn't fly with the sanctioning bodies we deal with. They tend to call out the minimum area as well as thickness allowed (typically .125" and a 6"x6"minimum area) and would reject it with a dimple die stamped hole. Just something to think about and maybe get clarified before you get much further along.
It's an awesome project though, good luck with it.
What exactly is going on here?
Much of this car is sheet AL and is in the way of the planned cage bars. Before removing too much of what I want gone, I'm adding a brace between the shock towers.
Good progress today: Passenger side A-pillar bar is done and tacked in. The miters where these bars meet the halo are a pain. They should be mirror images so templating the tube and miter made a bunch of sense. The pics pretty much tell the story but - cut to the same-ish length, wrap a piece of paper around the miter and trim to shape, reverse the template and fit it to the other tube. Then do a "pretty close to the line" trimming before checking fit and doing final tuning. Thank the gods of fabrication for fixturing tables. I feel like I'm cheating every time I use it.
Use the completed tube as a guide for the initial cut.
Completed miter tempate.
Flip the template over and position it CAREFULLY on the other A-pillar tube. Chant and do prayer to the gods of fabrication that you got it where it needs to be. Then CUT IT!
Then it's the "check fit, tune, check fit, tune" until complete. Tack it in then fab and tack in the corner braces. These were straight forward and went pretty quickly. You can see the main hoop rear supports have also been tacked in. Again, pretty simple cuts and fitment. Note that the rear supports' attachment points at the rear cross bar were not tacked as the cage will need to rotate down to facilitate welding everything.
It's actually starting to look like a track car. Well, as much as a giant MB coupe can, anyway. Gotta say, I'm digging it!
Another view. Blue tape to help visualize additional bracing ideas
I can't wait to see this thing rip. I love big cars on a diet with lots of power!
Today's effort was the main hoop rear supports. I opted for a central start point on the main hoop for reasons that will become clear as the build proceeds. Same as was done with the A-pillar bars: get one side fit then create a mirror image. Here's the initializing of the mirror image.
Did I mention how much I love this fixturing table? 
It allows for things like this: duplicate the target dimension of the chassis and "check / fit / check / fit" the central tube point without having to put the tube in and out of the car a jillion times.
Who-Ray! The miters turned out nicely.
Rear support bars tacked in.
And another view of today's effort. Now, it's on to the same deal from the front of the main hoop to the halo bar.
Finally, a simple tube! Dash bar fit. And yes, the Benz steering colum will be replaced with something suitable. 
Main hoop-to-halo gussets done. It was hot, hot, hot in the shop today so this was all I was able to finish before it got even more uncomfortable.
'nother angle.
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