I'd say biggest problem will be the backspacing that comes with a 6" rim, will probably rub the crap out of things on the inside. othewise, its not ideal, but I have done 12.50's on 7's with spacers and no real problems. Definately helps keep the bead seated at low pressures, thats for sure....
Also, I got a set of 15x8 aluminum sawblade style wheels, would let go for $100, if its a Yota, or 6 on 5.5