So it sounds as though you're thinking of using the 9' bed with 17"single rear wheels instead of 18" dual rear wheels. Does your GMC have the original front hubs that came on DRW trucks? I don't think the 17" wheels will work with that original hub as the offset will be wrong but maybe you could swap them out for the ones on the parts truck.
Fox's custom bed is an inspiration but I was thinking of slightly different way to go on my '48 1Ton, namely a standard 9' bed widened to cover the DRW. That way you can use the original rear fenders without having to fab up custom wider ones. The truck currently has a military box on the back about 7' wide x 9' long and like the proportions as well as the capacity. Everything fits in there!
