Ideally, the fiver would pull level; and ideally, you would have >6 inches clearance between the top of the bed rail and the underside of the fiver overhang.
But, how much nose high is too much? I had the same problem when I bought my new RAM. My bed rails measured 59" from the floor at the back. But after lowering the pin box and loading the truck, I'm only 2.5" higher in the nose than I would have been. The pin weight tends to level out the gap because the truck is higher in the rear of the bed than the front of the bed. I also put on a set of higher profile tires which the fiver needed anyway. This gave me right at 5.75" of clearance between the side rail and the fiver measured at the rear. Honestly, I wish I had more clearance (>6.5" would be nice), but I haven't had a problem at 5.75".
Is 2.5" high out of level too high? I don't think so. The specs on the refrigerator require within 3 degrees of level. That is 5 inches in 8 ft. With the refer mounted in the middle of the fiver, I'm well within specs. Does it pull differently? I can't tell any difference between this and my last truck which allowed it to pull level. I would think raising the fiver 3" with channel may affect the stability as much.
For these reasons, I suggest - buy the truck if it's a good deal (you'll love the Cummins engine). Lower the pin box a notch, replace the tires if the fiver needs them, load up for camping and see where you are before doing anything, and especially before you decide not to buy the truck for this reason. You WILL find some solution, and you might decide that pulling a LITTLE nose high is OK.