I've done just about everything that everyone's suggested here. Anything from bolt extractors to actually welding on a nut to the the stud itself. I've used rust penetrating oil, I've heated the bolt. Nothing has worked. I've finally decided to just put in new threaded inserts, but since these bolts were holding my fender on, I was wondering if it's safe / a good idea to just cut off around the bolt and weld in a new piece of metal and drill it out for my insert. I'd like to have the bolts in the same spot for aesthetic purposes and also so I don't need to start drilling holes in my fender to match up. I should probably also mention that the metal is sort of warped around the bolt, but you can't really see it well in this picture. Also would it damage the frame significantly?

 

ur going to need to drill the bolt completely out and run a tap back thru it.

 

Either drill out what you have there, apply heat from a torch to break the bolt free, or cut and weld in your patch. I assume if you are welding in a new patch you can weld on a captive nut?

 

Yeah, I've tried just about everything and because I broke off a screw extractor in it, it's virtually impossible to drill a pilot hole. Welding a patch is a last resort. I've never done anything of this sort before though. Is it as simple as drilling a hole and tacking the nut on?

 

A carbide centerdrill will take out (drill through) the extractor. Might have to look for an industrial supply catalog/company. They aren't cheap and very brittle. A number 2 center drill might be about the right size. You spin it as fast as your hand drill will go being careful and percise. You might get enough of it out that whats left loosens up. Then, I'd try a blow gun to lift it out. Also, next time try a square type screw extractor instead of the sprial type. They dont snap as easily.

 

If there is access to the inside of the frame rail in that spot (cant tell which blind nut this is, but the frame rails behind the fenders have several holes in them for the factory spot welders to gain access) then use a socket (most of the blind nuts on our cars have hex heads like a normal nut) to just break it off of the sheet metal. Then you can hold a new nut in its place and run a bead on the top side (where you are looking at in your pic) being careful not to get any crap on the threads. Grind the weld smooth and paint so it doesn't rust. Once the fender is on you'll never know it was done.
If you're unable to access the back of the nut then try to carefully cut around it with a dremel etc. With any luck all you have to do is weld a new nut in and not do any sheetmetal replacement at all, that would be ideal.
Good luck!