30x9.50x15 is the largest that will fit on an S10 4x4 chassis in stock form, 31x10.50sx15 will work if it's a ZR-2 off rad package chassis. If you want LT Metric, LT235/75/15 is the only 15" size available, it will be a 6 ply C load range. Anything esle will be a Passenger rated, and even then, 255/70/15 is about the largest you'll find.
Given the engine that you plan to run, I would strongly advise swapping in at least the rear axle from the late ZR-2 S 10, as they used a narroewd version of the full size truck 10 bolt, which is not only much stronger, but on trucks after '97, have rear disk brakes.
