The primary issue with attempting to use a 71-2 Mach 1 grille on a 73 is, the full length protruding plastic lip on the bottom of the 71-2 grille will interfere with your 73 urethane bumper. The second issue is, the hole in the top of the 71-2 grille for the hood release latch would have to be relocated/enlarged. Headlight mounting bucket points are the same for all and I’m sure any center support mounting issues could be gracefully overcome, but given the first 2 issues, I wouldn’t attempt it.
I’m not a big fan of the understated look of the 73 Mach 1 grille, so I plan on installing the standard 73 grille (with the large horse in the corral) on my 73 Mach 1.