on the 2012 gmc 3500 i had to add a 3/4 inch piece of plywood, the camper sides now sit 1/2 inch above the above the bed rails.
....looking at the HD 6'6" and 8' box drawings for the 2011 pick-ups, the height from the top of "floor ribs" to the top of bed truck fleetside (assuming no bed rail cap(s) are installed), is: 20.830 inches (and, not discounting the ribs, appears to be 21.270 inches).
I had to raise my 2008 Gmc Sierra 3500 for the last 4.5 years. My outfitter then sat right against the rear bed and eventially wore asignificant dent in the "tappered" I corrected the problem and bot a 2012 Ram 3500 SRW, fits much, much better now!
2004 Nuwa Hitchhiker DA 31.5
2011 Juno 10 Outfitter
2012 Dodge 3500 SRW 4x4 Cummins Crew Cab
Lund 19 ft Pro V Fishing Boat