A simple ( but complicated!) answer...if that even makes sense:
All 71 & 72 Mustangs of every model came standard with 14" wheels/ tires, and 15" Magnum 500's were available on any model with a V-8 engine.
The singular exception in '71 is the BOSS 351 which came with 15" wheels and 2-puece hubcaps ( an extremely rare setup, by the way!)
I am not positive if Magnum 500s were optional on Grandes or not, but all the V8 options were, so I think maybe they might have been. However I have never seen an original Grande with Magnums.
In 73, all Mustangs came with 14" wheels with a variety of standard or optional hubcaps, and new-for-73 14" aluminum rims were available optionally on Mach 1s and convertibles, and possibly ( but not positive) on other v-8 powered models.
There were no 15" wheels (Magnum 500 or otherwise) available on any 73 Mustang.