what about 3.75" BS...any better?
Mine are either 3.75 or 3.5... I don't remember, but mine don't rub, I'll check at lunch to see exactly how close they are. You may have a problem rubbing the wall of the inner fenders in the rear when your suspension is flexed, but rubbing that at slow speeds(momentarily) isn't going to destroy anything

I wouldn't worry too much about rubbing the frame, you can adjust your steering stops to fix that.
Another thing too - what brand/series tire are you going to use? Most 31" tires don't measure a full 31". My BFG AT's actual measurement is 30.7" brand new, my used ones measure about 30.4"