I've 'adapted' A.I.R. pumps before, for the same reason you mentioned.
Brackets, pulley flanges, shaft offset (stickout) and outlet elbows differ.
WAY easier to just get the right one.
Thankfully I don't have to go through that any more
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.