[HO455]

I seem to remember a thread about using a square body box with 67-72 fenders. You might look it up. Or maybe it was using a 60-66 box. I'm not remembering for sure but it's been done before. I don't believe there are major differences from your current bed and the ones before and after 67-72,. Most certainly less differences than the late model bed.
Years ago in the time before reproduction parts I made a new box bottom and front for a 51 GMC out of rectangular tubing, angle iron and diamond plate. I didn't own a welder then so I bolted the whole thing together like the factory did. It is still on the truck and working well today. All I had was a drill, a saws-all, a carpenter's square, some saw horses. Oh and patience.