Another trick is to cut it off, if it's brass this is easy, if you can open the center to get a small/thin sawzall or similar blade in there and cut perfectly level down towards the threads, do cut into the threads of the manifold just the fitting, and go slow and easy, as you get into the threads of the fitting you can try snapping it from each sides of the cut with a punch and hammer, it should snap and collapse, making the diameter of it smaller and it will simply unscrew by hand.