When I built my 48' flat bed I closely matched " L " beams to the thickness and depth of what tamiya uses. Then I just trimmed the beam down up near the king pin using the tamiya rail as a guide.
I kept the same rail spacing so I can use a stock tamiya tandem set up. This also let me use the king pin plate and support legs from the tamiya trailer.
http://www.rctruckandconstruction.co...ead.php?t=6992