What's the ideal backspacing for the 9 inch rim. I have 8's with 235's on the rear with a 4.75 backspacing, and it sits perfectly. The rear is pretty low(the tire is under the guard), however it is right on the edge of the guard. A 4.5 backspacing would hit the guard sitting this low.
A 9" rim with 255's with a 5.75 backspacing would have the same guard clearance as I have now, however it would sit 1" further inwards. A 5.5 backspace would sit 1/4" closer to the guards, and 3/4" further inwards. I may be able to get that 1/4" clearance to the guard, however I'd have to roll the guard a bit more.