I am making an assumption that this measurement is with tail gate up, correct?
So if indeed it will all boil down to the hitch height in bed vs. king pin distance.
I had a concern when I purchased my 2015 GMC because it sat a lot higher then my 2500. I put the B&W hitch at the lowest setting 17" and hooked up to fifth wheel. I am right at a 6 inch clearance of my bed rails. Now when I measured everything before hooking up I thought I was going to have issues. But all worked out fine.
The truck should compress a littl on the springs once the pin weight gets in the bed of the truck. That was the unknown piece I was not sure of.