How much torque can the stock 240z rear axle tack before it explodes? Also how much can it take a regular beating with?

You'll find that for the project you are considering, you'll get much better advice at hybridz.org, where they do this kind of thing all the time. This forum tends to be more about stock Zs, or at least people who stick with the Datsun sixes.

Arne's right - the HybridZ guys will have the experience on this one.

I can personally report that I ran a 331ci SBC in my '72 240 (that was built to match or exceed the specs of the Corvette 327ci/365 horse engine - don't know what my torque was) with a stock 3.36 R180 diff for more than 5 years. However, it wasn't really severe service - some dropped-clutch starts and much "spirited" shifting ... but the tires were usually the "safety clutch" that probably kept the diff from breaking as they would spin if you laid down too much power at the wrong time.

OTOH, a few years after pullling out the Chevy engine, and installing an L-28 six, I was pulling off the shoulder of a freeway, trying to accelerate into the traffic, and lost the diff during the 1-2 shift. After the tow home, I pulled the cover and about 1/3 of the teeth on the pinion gear had sheared off. Don't know if that was a manufacturing defect ... or the "debt" owed to the 5 years of V-8 service.

Goose

Here's a good place to start over there:

http://forums.hybridz.org/showthread.php?t=114798

A special thanks to jmortenson for this one.