[mr48chev]

When I enlarged the photo to full screen size it looks like the tops of the bed rails are spread out quite a bit. That could be from the truck having hauled heavy loads in it's lifetime or because there is no floor in the bed to add stability. I think it should measure the same distance across as the bed does at the front. I would measure the width of the bed about an inch from the front panel at both the top and bottom and then measure it at the back and see how far it is off and go from there. One of the guys who has a bed in perfect or near perfect shape might be able to give you some measurements to
help you out.

I've found by experience that these beds like to have every bolt in place and tight and every piece in it's correct place so that they will stay square.

Also remember that these trucks didn't always come that close to perfect from the factory and 50 + years of use and abuse take their toll. As Mylow53 mentioned, you can expect to do some fitting of the pieces to get it to what we consider up to par for a hotrod, custom or restoration these days.