[north]

There were basically 8 frames for the 68-72 A-G bodies (I’m not counting slight differences from year to year or divisions):

1... two door coupe or hardtop, 112” wheelbase, unboxed with short rear overhang (the hardtop version had some heavier gauge sections than post coupe)
2...convertible 2 door, same 112” wheelbase, boxed with short rear overhang
3... four door hardtops and sedans, 116” wheelbase, unboxed with short rear overhang (the hardtop version had some heavier gauge sections than the sedan)
4... regular wagons, 116” wheelbase, unboxed, with long (6” extra) rear overhang
5... “vista” wagons, 121” wheelbase, unboxed, with long (6” extra) rear overhang
6... El Camino, 116” wheelbase, boxed, long (6” extra) rear overhang frame
7... Monte Carlo, 116” wheelbase, unboxed, short rear overhang (difference from frames 1-6 is mounting points for cowl are 4” further back from front axle)
8... Grand Prix, 118” wheelbase, boxed, short rear overhang (difference from frames 1-6 is mounting points for cowl are 6” further back from front axle)

Note:

Frames #7 and #8 are unique because the distance between the front suspension centerline and the cowl mounting point is greater by 4 and 6” respectively compared to all the other frames (1 to 6) that all had the same standard distance from front axle to cowl.

The only interchanges possible:
#1 and #2 in that a convertible frame will bolt into a coupe or 2 dr hardtop (the convertible frame was an option on two door coupes and hardtops usually as part of towing package)
#4 and #6 interchange (though not offered by the factory as an option)