I was installing a new H pipe that I bought. I took of all bolts except the most difficult one. We tried heating up so it could give but it broke. SO what the way to remove the screw?
Put some liquid wrench on it and let it sit. later apply more heat to the threaded hole, let it cool, more liquid wrench. Repeat as often as time permits. Heat it up real good and try to remove it while it's still hot. It's better if you can do this over several days. Make sure you use a 6 point socket on it. Be carefull you don't snap the screw in the process. Use anti-sieze on the threads when you reinstall it and replace the heated screw.
Good luck.
Edit: opps, you got nothing to grab onto. overlooked the broken nut. You may have to remove the manifold. Your life is a bit more complicated now. Very little room to work there.