I was looking at the '57 (closest available) Ford truck brochure at the Old Car Manual Project, and the wheelbases were listed as 110" and 118" with either 6 1/2' or 8' beds, respectively. Looks like you would have to shorten the wheelbase and front of the bed 8mm, and the rear of the bed 10 mm, to get a correct 6 1/2' shortbed.