the diff that came with the 4cyl is different than the ones of the 6cyl, and the driveshaft is a different length as a result. For the least amount of headaches, buy a wrecked M3 and pull everything it has; that is, the engine, clutch, tranny, driveshaft, diff, ECU, radiator, suspension, and whatever else Im forgetting. It will cost less than buying each of the parts indiviualy, and when you're done you can sell off the remaining parts.
<span style='color:gray'><span style='font-family:Courier'>1995 e36 3 Series RIP
1999 e46 3 Series BURN IN HELL
2000 MR-S Roadster= Current</span></span>