Offset for 20's on 93 S-10

Was wondering if any body can tell me what offset to go with on a 93 s-10 that is going to be laying frame?? Im baggin this truck for a guy and he has 20x8.5's on it right now and they are hitting the front fenders as i let the truck down with the jack. So he was wondering if I could find out exactly what he needs to buy!! Any help would be greatful.