4" BS is probably your best bet with an 8" wheel. Anything greater will give you clearance problems on the inside, putting the tires too close to the rear frame rails in the rear, and reducing available steering lock in the front by having the tire sidewall rub the frame. Going less than 4" starts to move the wheels too far outboard, so you're looking at wheelwell lip interference. All my experience is with an El Camino. Do some searching in this forum and you'll find many comments of different tire and wheel sizes, but 4" BS still seems to be the safe bet.
Bill