hmm i guess my replies are a continuing conversation of sorts.

to hold 400 whp you'd need a new clutch, something heavy duty, probably kevlar. as far as the transmission internals go, you may or may not need to replace some of the components to keep the tranny from handgrenading over time. Final drive, countershafts, synchros, etc.

you might want to look into the H23 tranny if you're boosting, since they're geared longer (which is good and bad) and are also pretty cheap to replace.

good luck