Can anybody help me by choosung the right diff. I'm using a ZX-12 engine. What ratio do I need and in what car (year) can i find them.

IRS or live axle?

I can email you a gear ratio spreadsheet if you U2U me your email address - you wanna recon on a top speed of at least 120mph on the calc to get a good spread of gears and a reasonable cruising rpm.

You will probably need between 3.09 and 3.54.