I took a piece of 7" channel iron the width of the frame, drilled a hole in the middle the size of the shank on the ball. Next I welded the nut to the underside of the channel iron. I laid the channel iron on the frame of the pickup on top of a pair of heavy 4" angle iron about 10" long laid lenghtwise on the outside of the frame. These were drilled four bolt holes two per side. Squared up, clamped and welded the angles to the channel. I took a piece of 3/8" flat iron slightly wider than the width of the channel iron and long enough to fit between the angle irons, laid it on the bottom of the channel iron, took a measurement of the distance between the bottom of the nut on the channel and the bottom of the flat iron and found a heavy bushing(like a heavy walled pipe) that I welded to the nut, passed through a hole I drilled in the flat plate, squared up the plate, welded it to the channel iron along the sides, angle irons on the ends, and then welded the bushing to the bottom of the flat plate. Drilled the frame for the bolts and installed the plate. It give you a ball that can be turned out leaving the bed clear with a hole that goes through to let the dirt out or you can cover it. It has worked well for me. Cost about $50 to make.
Disclaimer:
This design has not been professionally strength tested. Writer takes no responsibility or liability for design or use thereof.