Actually a rear clip is what you are considering, and it really isn't that difficult except for the rockers...
Remove the rear window,seats, carpet, pull the headliner back(or out).
Grind out the factory filler in the sail panel between the back windows and the rear window, cut all the spot welds, same on the inside. There is a bunch.
Clean off the factory undercoating inside the car under the front seat where it has a seam in the middle, cut all those spot welds.
Here is where the pain is, the rocker panels have a lot of reinforcement in them and if you can slightly overlap the old and new, and weld them solid you'll be ok.
Get it all apart, get the new clip ready to fit with the rest of the body(straighten the edges where it joins, and start working it back
together.
Measure the Wheelbase! b4 and after until you get it back to where it belongs, fit the rear window in, check and recheck, measure diagonally to get it square(spring perch holes or any reference point b4 and after).
patience will get you there.
Yep it is a pain but doable, even by a novice, just may not be "worth" doing.
More work is to take the skins off, the 72, but I think the trunk is different inside, and things may not line up right.... hmmm