Ahhh...ok, thanks for correcting my confusion. Hope this helps others as well. For a stock guy like myself though, the older 180sx diff would be the way to go it would seem. Time to do a little searching.
I was considering the j30 diff just because they seem so plentiful, but as suggest my stock motor set wouldnt benifit from the 3.90 gear set. Except for maybe on long touring/ highway trips.