For breech plug removal I use a hefty old timey smooth faced adjustable wrench, you could get by with a larger modern adjustable, smooth faced. You might have to use a length of pipe to lengthen the handle and provide more leverage (pipe slides over the handle, maybe two foot long). Be SURE not to mar the upper part, top, of the Tang, clamp the wrench jaws below that area. It unscrews counter-clockwise. (Moderator correction.) The breech plug on Chambers (Sold by his daughter who's company is Buckeye barrels) are exactly 1/2 inch into the barrel. I too remove the plug and check for fit and and then polish the face. Polishing the face makes for easier cleaning as the residue comes off without much effort.
When setting up to drill the White Lighting hole carefully locate and punch dead center of the flat, level the barrel at that end so you have the bit entering squarely. Do drill all the way on that side of the barrel but when you tap for the threads stop before you go all the way through, this allows the liner to seat firmly but not protrude into the inside of the barrel. Hack saw off the waste then carefully file the liner face flat.
Good luck, go easy on that tang top, you may need to gently tap the wrench handle to get the plug moving.
dave