You are (doing something that's too complicated).
The issue with the rear bars - if they're bent, you need to go to the next size tube diameter than what's required.
For example: If you need a 1 5/8 x .120 tube if you bend it, it must be 1 3/4 x .120, or be a thicker diameter wall.
We have unibody cars. There's no frame or structure to attach the bars to - so the 6x6" plate is legit if it's welded to the floor.
you modify the firewall or the floor of the stock vehicle. If you do this, then you cause extra requirement & scrutiny upon yourself (requires a cage of some sort).
You don't need to open your floor up and drop the hoop onto the frame connectors. My rear hoop is directly over the front spring eyes, but this car is a 73 camaro. I'd assume the 1st gen bar would end up in the same spot.
I used to work tech at Road Atlanta, that don't mean jack squat, except I checked a lot of cars. From goofy VW's to Trans Am hardware. I took my time with the GT1 style hardware.