Try the BBS at www.s-series.org. Those guys run Nitrous, superchargers, and all kinds of wild stuff on their NEW S10s! One guy even swapped a 6 speed in his 98!
There are many hi-po parts available for the 4.3L Vortech V6, just how much you wanna spend? Good luck.
1989 GMC S-15 4x4 4.3L/700R-4, 3.08s, 31" Sport Kings For Sale
1985 S-10 4x4 Soon to have 3504BM/TH350/D44/14FF, 35" Boggers