Keith Kearcher in Bend, Oregon has fit barrels from one Lefever to an action from another for me twice. He created two mixed number but very usable guns from four piles of previously almost useless parts. Either he or Buck Hamlin have the skill to do what you want done.
I use Keith for almost all of my double projects as we live in the same small town thus avoiding the shipping risks. He called yesterday to say a recently acquired C grade which someone had poorly sleeved down from 12 to 16 is ready to be picked up after he cleaned up the sleeve joint and blued the tubes; we're looking for a set of 12 gauge barrels from a gun with in frame ejectors to make it into a two gauge gun (the 16 barrels are really too narrow at the standing breech, but are completely functional, just weird looking with a small flange at the junction).
