On most of the fenders where they bolt to a running board, that area is susceptible to damage, cracks, etc from the bolts into the running boards... The factory usually has a strip of metal at the bottom inside of the fenders to help strengthen this area and spread out any stresses, hopefully preventing damage from bolt stress/fatigue. The "remnants" of one can be seen here:
http://i5.photobucket.com/albums/y167/rmccartney/Mar%2010%202013%20OJ%20Higgins%20Meet/_MG_9996.jpg This fender was in decent shape and yet the inside strip was practically gone. If your fenders are rusted through in this area, it's likely the strip of metal inside is history. I'm not 100% sure of the thickness, but I believe it is the same as the fender's skin. Should be about 18 ga.
