Inner sactum oil galley plug

Update: A happy ending--I took the block over to my nephew (welder/mechanic) to use some heat on the threaded plug. However, he first insisted on seeing how tight it was by pounding away on it with a hammer and punch and trying to break it loose with pure strength. Sure enough, he was able to break it loose.

I had spent all day yesterday spraying the plug with Liquid Wrench and tapping it lightly with a hammer. My end result was half-rounding off the inside corners of the plug (the plug was not square like the factory plugs--it was 5/16" hex).