Many years ago I bought a tool chest for my then '84 Isuzu P'up diesel. It turned out to be perfect for the full size truck. It just fits between the bed rails and with two pieces of 4X4 wood post, it sits almost level with the bed rails. And to think it looked huge on the P'up. The front hitch rail is almost touching the wood that is even with the face of the tool chest. Anything bigger wouldn't fit. If I have the hitch in the front set of rails, the jaw release handle hits the lid if I don't move it out of the way before closing the lid. With the hitch in the back set of rails centered 12" behind the axle, I have about 15" between the hitch and box.
2020 F250 STX CC SB 7.3L 10spd 3.55 4x4
2010 F250 XLT CC SB 5.4L 5spdTS 3.73
ex '95 Cummins,'98 12v Cummins,'01.5 Cummins,'03 Cummins; '05 Hemi
2017 Jayco 28RLS TT 32.5'