TH400 doesn't direct fit and AW4 is easier to put in by far than a TH400 (physically). The flexplates are different for each trans but 9XX (904, 998, 999, etc) and 727 interchange until '87 when they changed the ring gear offset (10mm or 3/8") closer to the motor and went with the XJ starter. Another possible starter fix would be to find a '72 -80 IH Scout starter for 6 cyl auto. It's a Delco Chevy style starter that is way better than the Ford ones. Speaking of the Ford ones, most cheap rebuilds come with an actual Ford starter drive which is too short to mesh correctly with the AMC ring gear long term.
I haven't checked fit yet but Napa starter number 246-0674 "should" bolt right up. They use the same flexplate.
Correct length AMC starter drive
Sorry for rambling (even for a Rambler man) I blame it on the cold meds!