hello joe, my side mount is on a 55 first series, so it might not be the same. but the stiffener attaches to the vertical post , and the rail. the bottom stiffener goes on the inside of the lower part of the bed panel, and attaches to the bolts that are on the lower part of the wheel mount on the outside. this peice is a rectangular bar with two holes threaded in it. i hope this helps.
