Any 727 that isn't a lockup or came out of a motorhome or 4x4 will work. Only thing to look out for is if your running a factory type shifter you'll need the tailhousing that has 2 holes on the bottom for the shifter linkage bracket to bolt to.
I'm also looking for a small block 904 to replace the slant 6 trans in my duster. Will 904s from somthing other than an A body work? difference in tailshaft housings/Length? Searched but couldn't find an answer....thanks