Having a 72 and trying to use parts from a 77, I can tell you that some stuff is definitely different.
Comes to mind off the top of my head:
Motor mount pads. 72 are flatter/prone to crack. Mine are cracked on my 72, the 77's are taller and have an extra "leg" for support.
Rear bump stop brackets. IIRC, 77 is larger bolt pattern but look identical.
If your 78 had an in frame fuel tank, the brackets/crossmember may be just a hair different.
Emergency brake lines run through the crossmember on the 72, 78 will have them on the drivers side below the framerail.
Sway bar is different between the 2. The 78 may have rivets where the bar mounts on the frame under the core support crossmember and vise versa.
Brake lines are routes different. 72, proportioning valve is horizontal and I believe outside the frame. 78 should be verticals and in the frame. I may be confused though, but there is a difference.
I believe that is it. I can't think of any other differences.
