My Dad knew of a neat frame for sale, originally from a '24 Dodge. It had been a trailer at one time, so some disassembly was required to fit it into the F-100.
There were tons of rivets in the frame, here I am drilling one of them out. Later on we tried grinding them off, which was somewhat faster.
Here's the frame and body mocked up, this was just to snap some pics of the rails.
Making some progress - Richard has been helping with his expertise...