My truck was used by a sheep farmer that welded in angel iron and dimaond plate steel. when I cut the spot welds ont the angle iron the box started to drop. the wood was rotted and only supported the box as a hole across the etire sheet of dimaond plate. I supported the bed from the sides and cut out the metal and then rebuild the entire bed. I would bet that your bed sides are rusted out where the original angel side strips are spot welded to the sides and support the edge planks. I replaced the lower 8" of metal and then spot welded new angle stips in for my wood. It took a lot of work but worth the effort. If money is no objet buy new sides, I wanted to keep as much Jansvielle Ws steel as possible and money was an object so I replaced the bottem. Good luck.