I leave the 5th wheel rails on the truck. Using a straight edge, from the top of one rail to the other, I found I needed 1 1/8 inch of filler from the bed to the top of the rail. So I too made a foam plywood spacer, three of them as Wing mentioned. With half inch foam in the center an 3/8 plywood on top and bottom, it came out to the needed height.
My Lance 845 is reasonably light in weight, about 2,750 lbs, empty with options. Then beings I am a pack rat, I have to figure another thousand pounds by the time we hit the road. So far I am running unmodified on this truck. My previous Dodge truck had air bags, camper package, etc and it made it a good hauler but it was a 3/4 ton truck. I also like spring spacers on the rear and may add a set to my Chevy, especially if we trade up to a larger TC.
