you could try and use ph crack-it it comes in a aerosol tin and it has a freezing solution in it and you leave it for a few minutes then try to take the nut off and 9 times out of 10 it works !!
I will give both ideas a go, when I can get all the other tools :-)This looks salvageable to me ..
As has been suggested already ..
Heat the whole assembly (blowlamp) till smoking then throw Ice cold water on it ..
rinse and repeat several times and I'm 99.9% certain it will free up ..
A cleanup wire brush and repaint almost certainly neccessary afterwards..