I don't care for the long box, and short box are hard to find here. First thing we did was pull the box off and shorten the frame. We carefully measured the frame and took 14" out just behind the cab.
We used a removed section as the fish plate, and welded it together.
I cut 5" off the back of the frame, and moved the rear crossmember to the next forward set of holes.
New holes were drilled to mount the box.
Rearmost crossmember was moved to the next forward set of holes, and more hole drilling for the box mounts.
I could not find a good (or shortenable) driveshaft at the wreckers, so we had a driveshaft made.
To save bucks, I figured we could shorten the bed. I figured with some creative patching, we could patch the 14" removed from the front of the bed to repair the back of the bed.
I had bought some wheel arch patch panels to repair the rust, but then found a short box locally for not much more, so I took the arches back.
This box "looks" good, but magnets only stick in a few places. No matter. I can mix filler too.
This 81+ box had the main tank on the left side, my '77 had the main tank on the right, so we had to swap the tank to the other side. In removing the tank, kids had sliced the filler hose, breather hose, sender wire, and bent and broke the fuel pickup tube off the pickup. Because my truck came with a 20gal tank, I bought a 16gal for the shortened version. I also ordered a pickup with a return so I had the option to convert to EFI later.