Here is how my 70 GTX looks in the WORST possible position. Yes it flexed a LOT with the wheel cans removed but before the quarters and rockers were removed. We leveled the car on jack stands ,then removed the 1/4's, rockers, and pillar posts.
The replacement 1/4,rocker,pillar assemblys which were removed from a donor car went right on and fit with very minor effort. My next step will be to hang the doors from rebuilt hinges to make sure of alignment then I will finish welding all the holes. The pieces are welded and screwed (temporary) enough for clearance of doors at the moment.