255 will definitely fit on a 9.5 and look good. The tires will look wider on the back because the extra inch on the rim width will stretch the rubber out. It will also have a lower profile because the sidewall will be shorter from the tire being stretched. If you have two different size rims, why not go with two different size tires? You arent going to be rotating are you?

If you are set on keeping the 255, I woud recommend going to a 40 or 45 series on the rear to correct the tire height difference that will be caused from the different rim size.