I've never seen running boards on a 4000, 6000 56/57 truck, but per the parts books they all use the same actual running board 56-59. the only thing that would change would be the mounting hardware and fender extension panel.
I assume the 4000, 6000 Series had some sort of a brace to support the front corner of the board since the fenders weren't designed to have Extensions Panels, but can't confirm as I've never seen one (VERY rare).
Mike B
