remove the rear half of the inner fender and cut an access hole in the side of the frame. Use vise grips on the nut and be patient. If you are freaked out about cutting the frame, you can weld it back up when you are done. I have had open holes on mine for years with no extra rust and no trouble. I think it actually slowed the propogation of rust down by draining the water out of the area. The inner fender covers it completely, so only you will know it's there.
Removing the bumper is a lot of work, but you can go that route if you wish.
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1986 SE 350 V8