I have a th350 in my z31 with a 350 (if I ever get it done) I plan to use it to put tire tracks over the idiots who think a large chrome muffler dangling under their Honda makes it faster than a retty old 300ZX. I'm using it because I had it laying around and I'm on a budget. If I were to swap it out, I think the best choices would be either the 200R4 or 700R4. These are both pretty common GM tranys that should bolt onto a chevy, and give you a fourth gear. The 200 has a taller fourth gear (0.67)and will drop your rpms lower than the 700 (0.70) ther is only about 300 rpm difference at 85mph which IHMO isn't Jack. Either one should be fine. If your car was heavier I would suggest the 700R4 but as light as a Z is, and with your power, I don't see much difference. If you decide on a 200, i would suggest getting one from a Buick. I think they are stronger though I am not sure about that. Good luck.
Joe