if you concerned about a gearbox just bolt up a r5 turbo box to it, will take 250-300bhp and 250ft all day long, yes the ratios are slightly longer but you will have fair bit of power and they are pennies to replace.

on the diff front, i made my own lsd for my r5 box, i can show you some pics of how its done, you could probably copy it with a couple of hours on a milling machine