I assume you are talking about the fairlady or 'roadster' . I wouldnt have a clue about the ratio of the boxes in those things.
indeed the 1600 is a higher revver than the bigger motors eg : the 2 litre (and rare as hens teeth) U20. but what engine you put in it depends on if you are going to do a classic restoration or more of a streetcar thing.
the buick v6 you speak of is actually the engine we have in our Holden Commodores here in Australia. I would suggest you at least keep it in the family and go for the Nissan VG-30 ;) or better yet the FJ 20 / turbo , which would be neater and is the most commonly done.
here is a good site with a really neat example of a roadster , and some links to get you started off :
http://www.gordon-glasgow.org/gallery6.asp