Those sound very similar to the rails that were used to mount the
wooden floor of my dad's old Chevy stepside, and I suspect also Ford
truck beds that used wooden floors.  I don't recall at this date where
he purchased them from, but maybe you could inquire to one of the
manufacturers of repro pickup bed parts (Mar-K is one name that comes
to mind) and see if they'd be willing to send you a short sample of
the bed rail stock to see if it matches what's on your flatbed.  They
sell kits pre-punched with the holes for the usual configurations of
stepside pickup beds, but I believe you can also buy the stock without
holes.  You'd probably need to do that and then either punch the holes
yourself for a standard carriage bolt with a Greenlee chassis punch or
similar, or else find some bolts that look like carriage bolts but
have a knurled shoulder like a wheel stud so that they would grab in
round holes.

good luck,

nate

Check with LMC at lmctruck.com

Ted