I built one on a F-500 several years ago, used heavy 6" channel on the outside, hinged on the back of the frame. The inside was 2"X6" box tubing (thick wall) and turned about 14 rollers(more the better) that would just fit inside the channel. Drilled the tubing for the rollers off center about 1/2" so the tubing was above the channel a little. Cross members for the bed floor were welded to the tubing, the bed would slide back and forth in the channel using 1 cylinder attached to the channel frame at the front cross member and then to the bed cross member, this cylinder was about 7' long and 4" dia with a 2" rod(freebe). Mounted brackets to the truck frame in front of the rear spring hangers, mounted my lift arms there and to the bed cross members. When the bed was rolled back the arms lifted it, you have to play with the arm length to get it just right. Mounted a winch to the bed and went to hauling. Truck weighed 10,000 and hauled many 10-12,000 lb loads. Can't tell you any measurements because I built it on the fly and I copied some of it from another truck. It was light and simple, only had a 2 spool valve to operate.
