On the stepside beds, the edge angles are spot-welded at the factory, so they will have to be drilled or the spot-welds compromised, so they give up their hold.
All of the usual suspects, including LMC Truck, Classic Trucks, Jim Carter Classic Truck Parts and Mar-k, carries these stepside corner angles.
For the cost of the manufactured units, shop fabricating a pair, may not be the prudent approach. I believe they run approximately $30.00 per pair for the shortbed stepside edge angles, plus tax & shipping, from LMC Truck.