This was on the BB some years back, and was related to a stuck differential housing fill plug. That one is, I think, the same tapered thread pipe plug as in the tranny.

In that case a torch was used, very very carefully and sparingly. Not much heat needed.

Have at hand an ordinary cheap wax candle, the long tapered kind. Heat the plug only, it will warm the surrounding tranny casing enough. Something other than a torch might work.

While the plug is hot, apply the candle around the topside threads so that the wax melts. It will flow into the threads by capillary action.

Repeat several times.

Then use a Vise-Grip, 12-inch Stillson wrench (longer if it fits in there), whatever you have, etc. Idea provider said it works like FM (a type of magic).

Good Luck,

Bob

:>)