Jon, thanks for catching me on the tranny length and the driveshaft mod required. I have vague pictures in my head from "How to Modify and Race your Datsun," about the Z install into the 510, but not the other way around. Anyway...
Yep, I forgot about the length of the tranny--it is shorter, but it can still be made to work. Personally, I would do what the guys suggested above, and that's buying a known Z tranny. I find them often for sale on Craigslist in my area. If you do decide to install it, be prepared to have the shifter in a slightly awkward postion (farther forward). If the tranny if free though and you have your heart set on it, install it to see where the shifter comes in the cockpit. However, you will spend money on a new or modifed driveshaft anyway, so why not go for a known Z tranny anyway? It'll save a lot of time IMO. You are perhaps practicing false economy here.
Davy