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.