At 400+hp, What kills A904s?

Hope those snarky comments weren't directed at me... never said the guy needs a fully-prepped race trans, just citing it because it illustrates the 904 is an inherently strong design that can handle most anything you throw at it. Everything has a breaking point, throw enough power at it, you'll figure out it's limits pretty quick.

And as far as "building it right" - would you rather people on forums say just throw it together with offshore rebuild kit parts without checking clearances and run it behind a race motor?

The 904 I built for my Duster was well under $1,000 even with some nicer parts and an A&A "race prepped" case. 6 clutch front drum, 5 clutch rear, billet spring retainer, deep pan, T/A FMVB... borderline overkill for a 500 hp small block in an A body. Not much else to do to it unless you start rollerizing the whole thing, using lightened sun shells, billet this and that...none of which is even remotely necessary at his current level.