The suspension, that's all. Check it on Wiki compared to a GT. (PS, I've owned both)
Edit: tracking, wheel base, remap, extra torque, extra BHP, GTR brake lines, much desired body kit (check out the owners who've fitted the Nismo kit to a gt), and Mk1, non leather, track friendly seats.
Plus the extra strut brace strengthening.
And no need to fit Nismo badges, they come as standard
Plus, post #87 if your considering buying or modifying any new purchase on a Mk1 http://www.350z-uk.com/topic/105638-stang-thread/page__st__80
PM me if you require any further info