The best way is to mount the actual wheel diameter that that you will be running, (a plywood disc the same diameter as your fully inflated tire will work well here, drilled with your wheel bolt pattern and bolted to the axle flange with wheel nuts.) Check to see what your maximum suspension travel can possibly be,from desired ride height to "bottoming out the suspension and make your radius for the inside of your wheel tub equal to the maximum tire radius plus the suspension travel to "bottom out mode" plus 1 1/2". Just be really sure that your radius cut-out in the side of the bed doesn't extend beyond the area covered by the bolt on rear fender. If you find that it does, then you will have to go to some form of suspension travel limiter on it (a rubber bump block) so that you can decrease the radius. You will have to form or weld on a lip around the outside of the wheel tub to support the wooden floor.