What are the major differences between A904 and A727?

Their basically the same transmission except for the size of the parts. There are very few differences in them and they work exactly the same. The 727 was first introduced and then the time came that they needed a auto trans. for a 6 cyl. that didn't need to be as beefy so they made a little version of it known as the 904 or TF-6 as they were called in the old days. Later on they started using the 904 on small blocks.