The interchange on the hoods is only from 1976 to 1977. There's a hump on the hoods and header panels on the '73 to '75 LeMans. A '73 to '75 Grand Am may be usable but I'd have to check. NACA hoods should also interchange. There's a company that makes rear quarters for these year LeManses, but again, I'd have to research it (I'm typing this from work).
Apart from the boneyards, check at these forums...I'm on them and the people are knowledgeable and friendly, and many have spare parts.
Poncho Pride (pretty exclusively '73-77 Pontiacs)
The A-Body Site (all GM '73-77 A-bodies, though heavily Pontiac-oriented)
Performance Years (Pontiac-oriented, check the '73-77 Tech section)
A simple search should allow you to find these sights easily.