Upper control arms, torsion bars, rear springs, shocks, steering box and linkages, dash, and steering column. All of the electrical stuff. The wiring harness to the rear of the car will be different from 74 to 72 besides the body style difference. The gas tank and sending unit will work.
Front seat won't work because 4 door back rest does not tilt forward. The is a difference between the backrest bolt locations on the Duster than the 4-door. The backrest for the 4-door is also slightly wider. I think the lower cushion will work.
If the 74 still has the "seat belt" style door handle by the arm rest, all of the front door latch mechanism will work on the 72. (Also, the arm rest base and pad will work.) You may need a longer pull rod from the interior door handle to the latch mechanism.
I don't see a problem with the center portion of the trunk floor. The sides will not work as the fenders are different lengths.