I have a 56 Ford F-250 with overloads. It came with a grain box but I have removed that and am in the process of installing a flatbed. Can anyone tell me what size the original timbers were on top of the frame rails? I need height, width, and length.

 

I saw a 53' flatbed and it had 2 4x4's, one on top of the other. Be sure to use oak or treated wood. Oak stands up quite well for flatbeds.

 

Here's the 1954 factory stake bed/flat bed F-250 chart. The bold line of data gives load height for the different tire options. You should be able to do the math to come up with the bed support rail dimensions. Stu