904 vs 727

Thanks for the Pics; that makes sense because when I had the orig 71 904 and the 81 904 on the shop floor the converters would not interchange and I had 3 different non lockup converters to try from. The 81 was already build so I used it.
Do you know if the input shaft is the only difference? If so, can I just change that or does the pump need changing also?