Welcome!
Posted some gearbox/diff info here:
http://nissansilvia.co.uk/vbb/showthread.php?t=6158
However, if you're feeling adventurous, the RX8 6spd gearboxes and carbon fibre prop shafts can be had CHEAP. No idea if they're anywhere close to fitting but people have been sticking them in Escorts as a Type 9 replacement for a few years now - and that means adapter plates for various engines should be plentiful. F20C engine and an RX8 'box would be a nice first for an S12
