I would think you'd want to position the flatbed over the truck frame properly aligned from front to rear and side to side then figure out how to build mounting brackets.
The truck frame rails may not line up with the bed frame rails but if they do, four simple 1/4" steel straps welded to the outside edge of truck and bed frames on each side could hold it in place but you'll probably have to build small steel supports to fit under the truck bed rails to properly hold the weight it will carry.