I see a lot of flat beds built on old pickup frames that don' take into concideration tire clearance when loaded.
It is not hard to do. The simpleest way is to add feplace the studs on the brake drums, install a spacer between the drum and wheel for spring clearance, and buy the dished wheels. If you want the front to track right you will also need the offset spacer and dished wheel, otherwise it will not look right.