I see. The Studebaker bed, or if you're lucky enough to find one, a Canopy Express (basicaly, a Panel with open sides) is about as close as you'll get and still match the '51s body lines. Another option might be to check the salvage yards for a Panel, and get the parts you need. I almost did that myself with a Task Force 1 ton Panel that was missin pretty much everyhting from the cowl forward.
