The Ford part number is 34362-S, 5/16-18 Pierce Nut, 0.490 X 0.687 X 0.281 as measured. The mounting hole should be 0.490 X 0.500.
I believe that old man rust froze the fender bolt to the nut and an excessive amount of force removed the two together.
See the picture and chart.
Measure the hole in the fender; drill and tap a 1/4 inch thick steel bar or plate an appropriate distance from an edge to 5/16-18; using the tapped hole as center, cut the bar width to the hole width; grind the step to the hole length and panel thickness; cut the bar length 1/4 inch longer on each end; weld; grind; paint; have 2nd (or 3rd) beer. (Life is good.)
Jim