The 8x15 rims would be Ok. Do you have such a list for trucks until 1975? Which tire size had the Blazer in 1974/1975?
At the moment my C10 is fitted with 245/60/15 on 8x15" rallyes in the front and 295/50/15 on 10x15" in the rear. It looks really brute. Everybody likes it, but not the guys from the technical inspection agency
Best regards
Banno