Jeff Wisnia wrote: Uh, the OP did say those were gas tanks.....And there was "old gas" in the other tank.
^^^^^^^^^^^^^^^^^
The heating idea is so good that I am trying to think how to make it safe. Not only will heating break down some of the bond in the threads, making them release more easily, it will also expand the hole in the plug, so that on cooling, the Torx wrench will really the tightly held.

So, how about syphoning out the gas, and filling the tank with water. Or old crank-case oil, which will be miscible with the gasoline. Gasoline will float on water, and could still be dangerous, but diluted with oil, it ought to be pretty hard to light. Another idea is to run the exhaust from a small engine into the tank with a hose, and run it for 15-30 minutes. I know of a welding shop that does this when they weld gas-tanks.

Maybe try throwing in a few lumps of dry ice, which will fill the space with CO2.