If the header pipes have not been off yet, the finned exhaust nuts can be a problem .
You need a special tool to fit into the fins on the nut if you haven't done this before, if you try a punch and hammer, you will break the fins off .
If the nut moves, then stops, stop all efforts to remove the nut, cut the nut off and replace it, or you will be removing the head to have the threads on the exhaust port repaired .