Try using an acetylene torch with a SMALL tip. Heat the nut until it just begins to glow then loosen with your wrench. A mechanic friend of mine refers to this as a "blue wrench" and considers it a "must have" for just about any type of restoration work. Just be careful around fuel lines.
I have also had fair luck with a common propane torch used to solder copper fittings. It certainly does not produce the same heat as acetylene but might work in a few cases.
Good luck!