Now for the hard part.
There isn't much good information on the interwebs about how to shorten a 73-87, compared to the amount of info on 67-72 trucks. It's 14 inches. Make your own way from there. Best of luck.
I don't have a tractor with a loader or a tall shade tree, and I work by myself, so I had to get creative about removing the bed.
Just disassemble into pieces that one man can handle moving.
Then pressure wash to get a clean basis to start with.
The first step I took on the shorten job was to move the rear crossmember forward. Fortunately, GM punched the proper holes into the frame so it's an easy, no measure job.
You can just see the factory longbed location in the bottom of the picture with the rivets and the shortbed position with the empty holes I'm pointing to. It's a simple matter of removing the old rivets, sliding it forward, and bolting it in.
