If you're buying one of the old Z series, 240-280, then I would suggest something like a Chevy SB V8. That's the most popular swap for the old Z series. People say they drop in as if Datsun made the Z for the V8. If you're interested check out this site.
http://www.hybridz.org/ Other than that it would be a major pain trying to put a 300zx engine into an old Z.