If you can see the run out and the hole is at final diameter then just make new mono-block. It will be the fastest and best solution. Do you own a set of parallels? I would want to set up my piece so I clamped it with one side touching the rear jaw, the other side facing the movable jaw and the parallels supporting either the breech or the muzzle. Clamp it firmly and leave enough extra material so finish is not an issue.