Rule of thumb is that the rim be approximatly 2" narrower than the section width of the tire, with about 1" tolerance. So an 8.5" wide rim would call for a 10-11 inch wide tire. a 9" tire would be ok, though the wider tire would be better. The problem that will come up here is the chassis that is going to be used. The S truck chassis will have tire rub issues with 10" or wider tires on the front, they will rub on the front sway bar in the turns. The ZR-2 chassis used different control arms and sway bars, and can tolerate wider tires, but even then there's a limit, due to the lack of strength in the axles and brake size. If the wheels in question are a RWD style offset, you could probably get away with a bit wider tire than what would go on a stock S10 4x4 wheel. You will be looking at floatation sizes to get the width and load range you're looking for.
