The A body platform was essentially the same from 63-66 and from 67-76.
The E body platform was a totally separate program.
I understand your frustration. I have said it for years that when these cars are kept all stock, they are fantastic. IF you want headers, they are a pisser. Look at the design and of all the obstacles. The engineers somehow thought it made sense to cram almost everything on the left side. I understand the steering box and shift linkage but why put the starter there, the ATF cooler lines and speedometer cable? They could have spread things around a bit. Add to that the torsion bars and you see why headers cost more for our cars. The labor to design them to clear everything must have taken 4 times the amount of time to fit a 70-81 Camaro. The Chevy had the starter, cooler lines and the transmission downshift components (Vacuum Modulator) all on the right side plus the steering linkage was in front of axle centerline.
I'm not saying that the Chevy design was better, it just made things a LOT easier for exhaust clearance.
Here is a tip though.....Install the header and starter together at the same time. It is impossible to bolt up the header and slip the starter in place.