The hitch receiver is bolted to the U-channel that forms the rear frame of the pickup box and to the back part of the subframe that forms a support for the pickup box.
On top of the U-channel is a sill plate the covers the channel and forms a step upon which the box floorboards rest. This sill is normally riveted in place. When I re-restored the box in the late 1990s I didn’t have access to the tools for setting hot rivets so I clamped the sill in place with carriage head bolts. The fact that the sill can be unbolted made installing the hitch much easier.
The angle iron cross member at the rear of the hitch bolts to the front and bottom of the U-Channel. You will notice from the photos that I welded tabs to the bottom edge of the angle iron. The safety chain hangers bolt through the hole in the tabs and a partner hole beside it in the angle iron. These design serves two purposes: it anchors the hitch receiver and it locates the U-b0lt hangers. The U-bolts were repurposed from cable clamps.
I welded a piece of flat 3/8″ x 1 1/2″ flat iron to the top of the hitch receiver tube. 1/2″ holes were drilled 3 3/4″ apart. 1/2″ bolts inserted in these hole and tack welded to the strap form two of the three studs used to attach the spare tire under the box. A third bolt drops through a hole in a tab located on the front edge of the subframe rear crossmember.