While I might not agree with putting a V8 in an E30, I can commend the hard work it would take to complete a build like that. Still, do some research and find a BMW rear end that will handle the power. The M roadster/coupe had an S54 in 2001. Thats 330ish HP in the Z3 chassis. That might be the closet fit and power handling combination you can get for rear ends.