OK, thanks for the definitive answer. I thought that the famous crack occured at what is now the junction of the floor pan, and fender. I've never owned either, so it was an assumption on my part.
Common misconception. The crack occurred in the fender where it turned horizontal and went under the floor pan. Stepping on the edge of the metal pan could deflect it downward and crack the fender. The retro-fit supported the floor pan preventing the downward flex. It apparently worked as I've never heard of one breaking afer the support was installed.