You’ll need to find a gunsmith that owns and knows how to use a Mateo lathe in order to do it correctly. It’s not hard, I’ve done a number of them, but it does take a pretty good investment in schooling and tools to do correctly. Search for a gunsmith that has a metal lathe. You’ll need it to set the barrel cylinder correctly all the way round. I used to do this kind of stuff, but no one wants to pay for the time to set it up and do it the right way. Too many people want to pay next to nothing for the work. I gave up my FFL and now I only do gunsmithing work for fun and locally, person to person. Without an FFL I can’t accept work for compensation.
As for the need to be a licensed manufacturer, that’s an old myth. If you own the gun it can be altered by anyone with an FFL, minus the C&R variety. Now if you want custom work done on a gun before it’s sold by an FFL then you will need to have a manufacturer’s license.