I would submerge the breech end of the barrels in a good penetrant for a month before attempting the breech plug removal, Daryl. For a large volume of penetrant like that, ATF and acetone, mixed 50/50; is as good, and as cheap, as it gets.
In addition to the tool you need to remove the plug, you need to consider how you will hold the barrels in a vise to do this. It may require considerable force to break the bond free to start the breach plug unscrewing. It will require a solid, but safe, hold in a vise to make the barrels immovable.
