[fred cook]

If the heat exchangers are toast then use a sawzall to cut the down pipes. That will make it much easier to access the nuts. If you are trying to save the current exhaust, then penetrating oil and heat are your two best friends. Remember, you want to heat the nuts, not the studs. You might be able to heat the cylinder head enough for the entire stud to come out. The last thing you want to happen is to break a stud!