My 1 ton with dump grain bed had 1"x2" between the truck frame and the bed frame. I'm sure it serves as a buffer and as an anti squeek. I would not replace it with metal... unless you use a soft one like aluminum, brass, bronze, copper, etc...
The wood will be cheaper and easier to do. What type of wood? Mine was pine (old yellow pine, not the new soft white pine). You want a dense wood that does not shatter. NOT Oak or hickery. Walnut would be good, but expensive. Popler, birch, and maple would probably be OK.
