Either way, 4.1 and 4.3 are both less than ideal. You can import a new KAZ centre in your ratio of choice from Japan at great cost (around $1000) or fit the S15 helical centre and matching half shafts. I used to own an S15 and they hook up very nicely once fitted with a camber kit.
I spoke to Warwick last week and regarding diff ratios he told me to speak to the guys who fitted the 3.5 (or whatever) into Stiffy's Daytona... turns out he lucked upon that second hand.
Might be worth calling japanautos.com.au (with that eBay listing) to see if they have anything else. They're in Brisbane which is handy for you Merv.
Edit: the final drive ratio of the S15 diff is 3.692.
http://nissans15.com/spec.htm