I would run the 4.5 BS. I currently run 31x10.5x15's with the stock Gambler 5.5 BS. It rubs at full turn (haven't bothered adding the washers yet). I do think that at 3.75 you'll stick out kind of far, and 4.5 would be plenty to avoid rubbing. As far as upgrades go, I just got my Spidertrax 1.25 spacers in the mail today, and this will bring me to 4.25 overall BS with the stock rims. 33's go on soon. I suspect I'll have to add washers to the steering stops to prevent all rubbing with a 12.5 wide tire, but all the 3.75 BS wheels I see stick out too far IMHO.