I think the next question would be what is your budget for this project. That way an intelligent answer can be given as to whom should do the work.
As a starter, you might consider IT&D for your barrel work if you are not planning on a major (expensive) custom job. They can put a Douglas barrel of suitable caliber on your Mauser for a reasonable price and that you will be happy with.
http://www.itdcustomgun.com/
