Try a pneumatic (or electric) impact wrench tool in combination with heat gun. You will have a success in 99%.
Heat it up and let it cool down couple of times, but do not heat it up too much in first step. 2-3 times of 100-150 degrees of Celsius will be enough.
There's a drawback, too. You will need a new gasket.
On the farm, sometimes we encounter a stuck bolt. One quick and easy attempt to remove it is ( we used what we had) to grab a 3 lb hand sledge hammer and give the bolt a nice rap on the head. Bear in mind you do not want to hit your bolt or plug hard enough to break the engine case. Just sometimes a rap with a solid hammer would break a bond. A regular hammer might work just as well. Just don't hit it too hard, just a nice solid tap.
If the plug is recessed into the block, you could get a large punch or something solid to make an extension. 1/2" or 3/4" socket extension comes to mind. Just don't hit where you shouldn't and please don't hit too hard.
This method doesn't always work, but I have seen it and have done it myself enough that we will try it before the torches and other stuff come out.
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