I just replaced the same parts (rocker/step) on my '57 and found that the gap between the rear cab corner and step sill was significantly more than what came off (I got the Jim Carter version). I measured the old sill and it was, in fact, longer than the new one, so the gap was expected. Since this isn't a faithful resto I'm just going to fill the gap by welding in a strip of metal since there's no functional reason for the gap.
