The T5 in the S10 isn't probably much better for anything more than daily driving under v6 power. Most T5s - unless they've been modified/rebuilt - seem to be known to not handle a lot of power, but I haven't seen either way, a world-class unit would be better (there is wc and non-wc, neither are supposed to be very strong though). You may be better off trying the 4-speed, but you'll need the G-Body pedals and linkage (unless you go hydraulic), along with the G-Body-specific clutch fork/bellhousing. Does he have the bellhousing for this trans as well? Muncies/T10s/Saginaws all bolt up to the same bellhousing (and I believe they use the same crossmember), but you may need a different yoke/driveshaft length. Where are the selector arms on the trans? This should be an indicator of what type of trans it is, and the number of rings on the input shaft (running perpendicular to the splines) will dictate what set of ratios it has.