I have removed a lot of seized up bolts and with an acetelyne torch there is not a nut made on this planet that will not come free. Forget the stuff about both the nut and the bolt getting hot. Heat the nut to cherry red and twist it off with the impact. If the impact won't do it get a bigger impact .If that dosent do get more air. At the very least the bolt wll shear off and problem solved.