This is the "outrigger" style of 'cage building, a technique that works well for the narrow-set frame rails that are common on Ford trucks. A pair of "outrigger" tubes are set wide underneath the rocker panels, curving back into the frame on both ends. The cab cage tubes then land on the outrigger tubes. During the build, the cab cage tubing was tacked in place and then the tacked-together tubing was dropped down where it could be welded all the way around. Once the welding was completed, the cage was lifted back in place and the outrigger tubes were then welded solidly to the frame.