Not sure if this will be of any help in your situation. I have rebuilt several gearboxes and often times the end of the housing will be damaged in some way.
I clamp a huge screwdriver in the vice and turn the gearbox instead of turning the plug. I realize that everyone doesn't have a huge screwdriver, so if you clamp a piece of flat steel in your vice that will fit the slot in the plug, that will work just as well. Instead of turning it out all at once, turn it out a little and then turn it back in, turn it out a little then turn it back in.
Depending on how hard it is to turn you may need to use a pipe wrench for added leverage (put a rag between the pipe wrench jaws and the gearbox).
I know that you are concerned about damage to the threads, but if the plug needs to come out... then it needs to come out. You'll have to worry about thread damage after it's apart. I have done several that were messed up pretty bad, and luckily haven't ruined any yet.
Hopefully Mr. Louderback will be along shortly to offer his ideas (He's rebuilt 13 trillion of them, and knows EVERYTHING!).
This can be a tedious job so be patient.
Good luck.
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