There are some differences, but nothing you can't work around. If you wanted to put a 69 body on an 82 frame (or any from 75 and up), you would have to graft your old 69 rails past the rearend to your newer frame. I'm sure some tweaking is needed up at the front of the frame too, but I don't know if you could work with the 82s' front frame extension or not.
Transmission crossmembers...you should be able to swap automatics and standards w/o any problem. the older manual frames had a welded crossmember, automatics unbolted. You'd just need to weld a pivot stud on the frame for your clutch. I think at some point (75?) all cars got the removeable crossmember. They all had 2 exhaust holes up to 82.
Basically all the way back to 63 the frames are similar...body mounts did move between C2 and C3 generations, e-brake brackets did change, kickups were strengthened, the very fronts and rear are different throughout the years, and the rear frame sections of midyears have a distinct downturn to them as they meet the bumpers.