1978 Aspen Super Coupe, what's underneath them?

About the only differences in the F/M body rear suspension...standard vs. heavy duty...is the leaf pack. Regular is 4 leafs, HD is 5 leafs.
5 leaf springs came with the 8.25 only and all wagons were 8.25 with HD leafs and brakes, regardless of engine.
F and M are virtually identical under the skin. Same frames, floors, etc. Just varying differences over the life of the platforms.