There are so many possibilities to answer such a question, but if you want more power out of the engine, you won't find it in changing the diff or the gear-box.
Assuming the engine you've got is in good nick and properly tuned, you can't go past a set of extractors and a look at the carby (either something bigger or tuning/jetting the one you've got). Improve the breathing of the thing.
Then there is cam grind, dizzy regraph (and/or electronic ignition), head-shaving to increase compression, blue 12-port head transplant, super-charging, you name it. Spend as much as you want.
All the bits have got to work with each other though; there's no one miracle cure.
I reckon the best place for you to start is
Street Machine's Grunt Hunt series.
Hope that helps.
RET
