How much clearance is needed between the front of camper to front of truck bed. If my rubber bumpers touch the front of bed, I only have .5 inch at the top.
If you have 1/2" clearance between the front panel at the top rail and the front of the camper tub, I'd say you have enough room. Assuming that the bumper are compressed all they are going to. A 1" x 6" x 54"-60" board should be enough to give you more clearance if you think you need it.