you can mount a 72/71 on to a 73/74 and backwards without problem... body lines will match PERFECTLY... diff are:

-ONE bolt on inner fender
-higher cut on 73/74 opening wheel well
-front splash shield ( the one what bolts with valence ) needing to get the matching splash shield with fender. The area on 73/74 where this shield attachs is larger and "folded, covering lot more than earliers.
-Of cours the sidemarker, but that is just on 71