'75 Z with 4-speed had a 3:54 rear end. '81 (and '82-'83) non-turbo with a 5-speed had a 3:90. In answer to your other question, I don't think Z and ZX driveshafts are interchangeable.
Auto-trans ZXs had the R-180 rear end. I think (but am not positive) the gearing was 3:54. Only the manual trans non-turbo cars had the 3:90 R-200. Turbo ZXs had a 3:54 R-200. There is a pretty good discussion of this subject at the Datsun Garage website. Go to http://www.geocities.com/zgarage2001/z.html and click on 'performance tips.'
A forum community dedicated to Nissan and Datsun Z car owners and enthusiasts. Come join the discussion about performance, modifications, restorations, troubleshooting, maintenance, and more!