First of all it is not a dump bed. It is a wooden platform body with a hoist. Big, big difference. Not to make a big deal out of terminology but we might as well start by being correct.
Secondly, yes just replace the platform body. If you choose to make one then you've already got the pattern in front of you and it coule be built from kiln dried white oak for the structure and dense southern YP for the floor. Use screws and bolts and please..no nails.
Same desigbn will work if you use steel and the longsills should be 7/8" C channel, crossmembers can be structural 3-4" channel or fabricated channel. Use the dimensions on the current wooden body.
