Hi
we travelled from Houston Tx to North OK to pick up a big Lance----it was used on a 99? dodge 3500 dually---our truck was an 06 Ram 3500 dually, so we thought it would be a simple change over, but as you found, the over-cab sits on the cab roof AND the camper sides sat on the truck's bed sides. We just had time to buy a couple of sheets of 3/4 ply--which just fit the bed as 8x4, we travelled many thousands of miles in US/Canada like that---but on a couple of frost-heaves (spaced just right for max. effect) in Alaska (taken tooo fast) we pulled out a tie-down and hit the cab roof---we travelled the rest of our trip to England --via Galveston with a ratchet strap on one corner.
Since returning to UK we replaced one sheet of ply with a sheet of 2" polystyrene-------I put a 2x3 across the front of the bed (and cut 3" off the poly.)---------this has worked very well and has given us an extra 1 1/4" clearance.----And is easy to remove.
Nigel & Pamala