Trying to put in new plugs. 1989 Chevy truck 350 engine. Cast iron heads. Two of the plugs are stuck tight. When I put pressure on the wrench it just does move. It feels like the threads are fixing to gauld. Now, before I wring these threads out, how is the best way to remove these plugs? Also, What should I put on the threads so that I don't have this problem next time?