This is usually considered a job for a professional Smith with a working knowledge of double guns and how they lock up. The old smoke and fit method, very slowly taking off of the lump until the breechface touches the standing breech and the double under bolt begins to seat into its recess. If you do not have any experience with fitting things to extremely close tolerences, I would not choose this as a first project. If you are looking for someone to fit them for you, I would be happy to do it.
Steve