How strong is your impact wrench?
they work much better with no adapters and a snug fitting socket.
run it for about a minute in each direction, then a real long time in reverse. If the nut isn't hot enough to burn you, something is wrong with the wrench.

I favor the oxyacetylene torch...
heat the stubborn nut to brillant yellow orange (and starting to sparkle).
by the time you set the torch safely aside, and get your impact ready, the glow should be down to dull red, but still glowing.
Impact in reverse will put new threads on the nut at this point.
Try not to touch the hot nut while you put out the burning undercoating.
I used to do this a lot when I lived on the east coeast.

clean the exposed threads of the stud with a wire wheel before attempting removal.