if youre looking for gas mileage you have no need for a solid roller 350 in an s10 that is in need of traction bars. 3.73s are the next big step down from 4.11s, but if its a solid roller that was already built, the cam is probably going to get its most gains with a 4.10 gear. id find out what tranny is in there (maybe a th400?) and swap it with an a3+1 so you have an overdrive gear.

by the way what are your plans for the truck? daily driver or a race truck? thats the main deciding factor in what to do