The welding a nut on an old bolt usually works great, it not only gives you a new hex nut to put your wrench on, it also breaks the rusted bolt free from the threads. Charlie you do have a bigger problem with a flush mounted, slotted screw. You probably can't use an impact type screw driver because the hood is too flexible, if you are not worried about the paint, I would try the heat method. I have used a brazing tip so the heat is concentrated on just the screw or bolt but it will still usually burn the paint off the surrounding area.
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Ed