At 400+hp, What kills A904s?

OP, do you know what year your 904 is? They're all basically the same but there are slight internal differences between earlier and later years, especially after 1978 when they became lockup style. The lockup are generally less desirable for performance applications but like the video posted above, they can be made to work.

If you can find a way to put a wider 5 clutch front drum in your unit you will be ahead of the game, more holding power. The low gear planetary sets from later units are a popular addition as well, helps torque multiplication off the line. I have held the belief that the low gear sets were installed in the later units ('78-up) because the cars of the time were heavy and under powered and needed all the help they could get to get them rolling with any alacrity.

904s respond to buildups just like any other transmission. Use quality parts and pay attention to detail and you'll be fine.

If you believe internet hyperbole, a 904 is a tenth quicker in the 1/4 mile because it has less rotating mass than a comparably built 727. They take less power to turn.

Look up "Proflite" transmissions. Basically they're 727 cases with 904 guts. They're very expensive, semi-pro level units that use mostly aftermarket parts but the idea is still the same, less rotating mass.