I haven't used the aftermarket sills but i'd bet dollars to donuts that they're made from the same thin metal as the rest of the panels. If you cut a section of your old sill out, your local sheetmetal worker will be able to roll you some from THICKER material, which will resist rust for longer and stiffen the body of your ute.
Or you could buy a new one and give that to your sheetmetal worker as a template.