I'd say the oxy-acetylene might be the next choice, but considering. Try some penetrating oil. Gibbs, Kroil, PB blaster, there is even a recipe to make you own here on the board. Let it sit overnight, use a 6 point socket and a cheater bar with restraint, the object is to put as much pressure on the nut with the least effort. If that doesn't work gently heat with a propane torch, more penetrant, and let sit again. Try again, if it doesn't move, then try tightening it, you just want the the nut to move as soon as it does, back off. Try to remove it again. If that doesn't work. the oxy-acetylene maybe your only choice, I sure hope not.

Good Luck

Larry

I don't own a vehicle that isn't old enough to drink.