fester - The below screenshot taken from the
1959 Chevy Truck Data PDF shows those spacer boards sitting on top of the steel frame rail and clamped on by the U-bolt and clamp hardware. It doesn't provide specific dimensions of the boards themselves, but if you look closely through the vehicle height data below the pic (in the PDF itself), you should be able to do the math and figure out how tall they were. The width of the boards should match the width of your frame rails and length fit to size of the bed space available, plus however much overhang at the rear you intend to have.
I've also attached some extract pages from a 1955-59 FAM that depict the platform bed with support "boards" and some of the attachment hardware.
Note the measurements and diagram apply to 3/4 ton and up flat beds, so slight changes to fit your 3100 frame would be needed. You will also need to consider the rear axle frame "hump" in your support boards. Hope this covers what you need.
