My truck is a cab and chassis, wb is 176", 4" longer than a 250/350 CC long box, there is a 4" filler bolted to the front of the box.
The frame width is something like 4" narrower and flat on top, so to mount the box there are mounts welded to the outside of the frame in the positions where the box bolts are, some are taller than others so the box cross braces will sit on them correctly.
My truck was converted out in Ca, by a company called Roll-A-Long