Do what mauserdad said. The only different thing that I would suggest is using Kroil oil. I have a 71/84 that had a screw that would not come loose. This screw could not even get scratched so I spent 4 days off and on putting Kroil on it then just sitting there for a while tapping the top of the screw with a screwdriver with no handle. Kinda like a screwdriver punch. Anyways tapping it sets up vibrations in the metal so the kroil can seep into areas it normally would not go. On the 4th day she came loose and not a mark on the screw. I would be concerened with the impact tool snapping the head off the screw then there goes your serialized screw then you would have to drill the rest of it out. Just dont get in a hurry, it will come out.