Re: Need advice on removing a stuck spark plug
I have seen plugs that didn't want to come out because they had carbon on the threads right near the end (towards combustion chamber). I screwed them back in a bit, put penetrating oil on the exposed threads, waited, then screwed the plug in and out, a little further each time, rather than just muscle it out. May want to use a spark-plug tap to chase the threads in the head before new plug goes in.
I have no personal knowledge on the 300-6, but we have some knowledgable folks on this engine on the Late Bronco board. I'm sure they read this board too, so maybe they can tell you if this is common on this engine, and if they know a better way to get that plug out.