Mark, have you considered sawing the existing tang off on a line parrell to the bore and brazing a new tang in place after you have shaped it roughly?
If you have a breech plug tap you can drill and tap a bar by just bending an angle on it. Keep a dipper in your water bucket so you can keep cooling the thread end as you work.
Or, you could borrow one of Ian's barrels and use that to hold your tap :-)