What are the major differences between A904 and A727?

For your purposes, the valve bodies interchange between the 904 and 727, but most other stuff does not. That 727 book is pretty useless for cross-compatibility or even working on a 904. It didn't help me much at least.

That tech download is a lot more useful, but obviously no good for beefing up your trans for performance use. I'd get with someone knowledgeable when it comes time to order parts. A&A or Cope Racing Transmissions are a good place to start.

The 904s don't seem to blow up like the 727 has a reputation for (probably due to the weight of the parts as the design is the same).

Good luck:

Steve