What are the major differences between A904 and A727?

In stock form a 904 has no problem handling 300 hp and with just upgrades to the friction linings and the installation of a good shift improver kit will handle 400+ hp. The book you have is very informative on torqueflites whether a 904 or 727 since they work the same. Only exceptions are the clutch pack springs and way they take up the end play thrust. And as LX said the only thing that really interchanges is the valve body. Everything else looks the same but the 904 parts are smaller.

69Dartconv. has a good point on how much extra power it takes to spin a 727. That's why I always use a 904 until the HP power gets above the practical level of it's capabilities. You can change a bunch of parts to make it handle 600 hp but that gets expensive pretty quick.

LX guy the reason you don't see as many 904's blown up is probably due to them having a bolt in sprague from the factory. An upgrade necessary to the 727 when building a perf. trans.