My '48 3800 1 ton pick up (9 ft Box) had the rear frame extensions riveted on. Factory set up. I made this truck into a rack body 20 years ago , because the bed sides were too rotted out. Finding good 9ft bed sides is quite a challenge. 1 ton frames differ from 3/4 ton frames. There is no hump in the frame over the rear axle on the 1 ton frame.