I would probably get your box mounted on the frame. Put your running boards on. By having the running boards in place then you know the location from front to back. Then you need to get your height, line up the fenders where they should be on the running boards. Then you'll know the height at the front of the fender.
Does anyone know if the fenders are mounted in the same location as the 41-46? If so, I could give you some measurements off my truck.
