Had the "rounded corners" problem when changing from a 1991 1500 to the 2013 3500, both long bed Chevy's. Screwed a 4x8 3/4" hard wood plywood to the bottom of the camper ($50); and then, screwing treated 2x4's frame ($24) to the plywood. This raised things up about 2 1/2" (1" above the rounded corners) with about 3/4" clearance side-to-side at the end of the truck bed.
I also had to deal with both front & rear jacks being too short to raise the camper because of the higher ground-to-bed measurement of the 2013. Fixed this by building a 4x6 platform for the jack stands.
Having resolved the whole GVWR thing by going with a 1-ton ... my way of dealing with the changes to bed was to suggest a new camper for the new truck ... wife said: "yeah right ... go figure it out ..."