There is a rectangular capture nut that is inserted into the lowermost cowl/rocker that receives the fender nut. This was not one of Ford's better ideas, as the nut is not terribly secured down there.
When Midlife was taken down for restoration, those capture nuts were missing, and the fender was brazed to the rocker panel to keep everything in alignment.
When looking in the catalog for these things, look for cowl to fender capture nut or cowl to fender nut. They are cheap, and I've never seen one actually fit securely in the cowl. One usually has to lever the fender against the cowl as you're fastening the bolt to ensure the capture nut doesn't fully go into the cowl and allow you to spin the bolt without tightening it.