65 Barracuda daily driver

That's great that you have the correct transmission to swap out! I think once they went to the larger input shaft, they stayed the same until the moved to lockup converters in the 998's and 999's. Swapping to the later input shaft in the 904 may be a question better asked in the transmission forum section of FABO.
Were you able to snag some motor mounts from the dude in Louisiana?