Distance between camper rails and truck rails, how high can I go?

Building up my long wheelbase 550 with a uhaul box on it. The box has steel tube rails and the 550 rails are flat. It currently is bolted to the frame with 4 brackets and there is a 2X4 between the two rails.I would like to increase that distance by putting a wood 4X4 in its place. This will lift the box and additional 2" higher. This will give the clearance under the cabover to insulate it. I am going to put a 1 1/4" angle iron frame under the cabover, fill it with spray foam and cover it with aluminum. This way I don't loose head space with the bed inside. There are side plate that are welded to the box frame that hang down to keep the box centered between the frame rails. Should I weld a 3" or 4" tube or channel on the box rail then add a 2x4 in? My current ht is 9'. I only plan a 2" lift and then not sure about tires. Height isn't an issue. I want more clearance above the wheels. I will also spray foam insulate the underside of the box.
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Expedition Leader
I dont see any issue with that as long as the side loads on controlled by the extended frame rail plates. Make sure good hardware is used in the bolted locations as well.
Should I use grade 8 bolts? Also I was thinking about putting a heavy spring on the fromt mounts to allow for some flex? Box is coming off soom so the rails can be sand blasted, and the truck frame too.
Looks familiar. :mad: I think Uhaul builds them with the rust already there. :D I would use a minimum of grade 8 hardware. If you are allowing it to move in the front, you will need some guide plates to keep the box centered over the rails. Do you have a build thread going?


I do have a build thread going. I'm not very far along though. I spent the winter planning and acquiring lots of Parts. Spring is finally here and I'm back at it! I'll bump my build thread up