What does he do with the existing holes on the Right Side?
I think if he wanted to use a GM replacement barrel, he would have to ask the company NOT to drill it for a RH drum or vent. If they send a RH barrel, he will have to cut it off at the breech, and rethread the breech for a breechplug. If the barrel is already cut on the bottom flats for barrel keys, the keys will have to be relocated also. And, if using an existing T/C stock, the underrib will also have to be cut and shortened to allow the forestock to fit behind the underrib.