My grandfather used exterior grade plywood in the back of the 58. To him it was a work truck, and he would have never thought of spending big bucks to make a pretty looking bed. he just wanted something that would be functional. It looked decent, certainly a whole lot better than the worn out pine boards underneath it. If you plan on using your truck for hauling, park it outside in the weather, and don't plan on taking it to shows, plywood should be fine. I do kinda' like the idea of the composite boards in terms of durability, but then your looking at more money than plywood once again.
