Most of the problem areas have been mentioned above, but here's some pics that may help:
Yellow - outriggers are one of the first spots to rust, particularly closer to the sills
Red - sills - structural rust here is a deal breaker, will cost plenty to fix
If you can check inside the car, under the carpets on the inside surface of the sills, this will help
Pink - floor pans - these look pretty good in the pics, original stampings visible, check for patches or places obscured by thick undercoating. Check the seam areas where they meet the sills
Blue - footboxes - check where the floors kick up to the firewall, early signs of rust will appear here
Green - crossmembers - tend to get dented when used as jacking point. rust is bad, dents you can live with if not entering concours
White - Main frame rails - check everything as much as you can with a sharp awl, any rust through or thin spots are obviously bad
Pink - lower fenders - look for filler along lower edge and bottom
Orange - outer sills - " " "
Yellow - dog leg - repairs made here are generally with a patch panel. Look for lack of wire at the lower rear edge to see if it's been patched One of the first spots to show rust
Blue - rear fender - also a spot for patch panel. Look for the wire on the front edge to show a repair.
White - fender seam - electrolytic action between the aluminum shrouds and the steel fenders will cause corrosion showing itself by bubbling of the paint next to the stainless trim strips
Red - These have been repainted originals. Check to see if the ridges on the hubs are sharp or dull. If they're sharp, hubs and wheels will need to be replaced . not cheap.
<span style="font-size: 14pt"><span style="font-weight: bold"><span style="color: #FF0000">The short answer - CHECK EVERYTHING!</span></span></span>