You really only have a couple of options there.
1. Continue to wick penetrating oil down it and get the best stuff you can find like Kroil (good stuff), PB Blaster(comes highly recommended, I have never used it) or if there is a GM dealer near you see if they have a can of 1052627 Rust Penetrant and Inhibitor, Heat Valve Lubricant on the shelf. I have been using this stuff forever and it works well too. Let it soak for as long as you can and then carefully work the plug back and forth the 1/3-1/2 thread that you have. With any luck the penetrant will get in there and free stuff up and you can work the thread out.

2. Just unscrew the damn thing and let the chips fall where they may. You will need to put a time sert or heli coil back in the hole when done. A time sert done right is about as good as it gets and I would like to think that whomever fixed it last time did it right and it is now just a case of another hole gone bad and not the same one twice. I seem to have more of the #3and #4 holes pull this stunt than the front two.

Which ever way this turns out, make sure that you put a tiny dab of anti-seize on the plug before you torque the new one back in the hole.

Mark