Spark plug gap = .025-.028. What I did for the scratches, I used the back side of a piece of mirror and laid down a sheet of 120 grit sandpaper, then placed head flat on the sandpaper and rubbed the head back and forth until the scratches were gone or substantially reduced, maybe a couple gouges at the edge that wouldn't sand off. Then I used a sheet of 320 grit sandpaper and polished up some more. Doing this along with anew head gasket will greatly reduce your chances of blowing out the gasket. My current engine had one side of the head torqued so tight that it left a huge impression in that side of the gasket, for sure that was going to leak. If you have access to a thin sheet of copper, then I would recommend cutting your own gasket from that. That will give you more compression and even more if you use the NGK plug.