measured the thickness of the repops and found it to be the same thickness as the originals.. so you would have to think that the metal is just more flexible. This could be because

modern metal is bent so that the bends offer intergrated structural integrity across the total area. (There has been megamillions spend in research of how metal can be formed to offer the most strenght from the the lightest guage metal.) Earlier cars didn't take intergrated structural inegrity into account; as a restult, the antiquated design doesn't offer intergrated support.