I finished putting the aluminum cap off my 99 Dodge on my 2013 about 15 minutes ago.
The major problem comes from the rear door overlaps the tailgate on the older truck. The 2013 has a weird rounded shape and a much thicker tailgate.
I was able to work around this by using 1/2 inch plywood strips (painted black) on the side rails and front to shim the cap up higher. I also removed the plastic trim that was on top of the tailgate. It had a ridge in it, and replaced it with a piece of rubber trailer roofing. The back door of the cap now clears the top of the tailgate and the rubber strip on the bottom seals reasonably well to the flat top of the tailgate.
A little crude perhaps, but it doesn't look too bad and let me use the cap I have while I recover from the shock of the price of the truck
Rick Davis
99 Dodge Cummins 576K
2013 400 miles