A rebuild kit for the A500 (with premium clutches, bands and kolene steels) is around $700-$800+ a set. A shop quoted me $1300 to rebuild an A500.
A junkyard trans is $145.
I've never fully rebuilt a 904 or 727, either, but with all the "dope" around the www, the ATSG manuals, and shop manual, I'm sure gonna give it a heck of a try!!!