...SNIP...... I agree, I do need to get the block very hot around the plug before this guy is going to budge. Without knowing if this is a solid plug or something weaker I am concerned about ripping a hole in it and then be in a world of hurt or worse yet damaging the block itself. Always looking for additional suggestions!
The problem with heating the block in that area with a torch is that the block itself resists the expansion of any local hot spot.
I assume you mean an oxy/acetylene torch with a heating tip. Propane or Mapp gas isn't going to do enough to be worth the effort.
Instead of heating the material around the plug with any kind of torch, I think that you would gain more expansion of the block in the plug's tapered thread area by simply starting the engine and running it until the entire engine block is very hot. You might need to cover the radiator to get it as hot as you can without hurting things.
At that point it wouldn't surprise me if the plug could be unscrewed with normal force. If not, then you can still use the oxy/acet. torch to put even more heat into the local area of the block if you want.
The Yanmar plug itself is a tapered thread, so it will break loose more like a plumbing fitting than a nut and bolt. However the plug and block may well be a 55 degree BSPT thread shape often used on Japanese equipent rather than the more usual 60 degree NPT thread angle that is common for plumbing fittings in the United States. Some are, and I've also heard that some are not.
But if yours is BSPT, you don't want to put an NPT plug back into that hole in the block even though it very nearly fits (same number of threads/inch).
You can get a BSPT plug from some sports car shops, McMaster-Carr, Hoye tractor, or most any Yanmar engine shop.
rScotty