Some say dont weld on a barrel shank, some you can. I feel the best way, would be to cut doen the shank a bit and make a 2 thousands under bushing, heat and do a press fit.then weld the bushing to the barrel right where it touches the barrel shoulder
Well, if you weld on a barrel shank made out of 4140, it will be so hard you may not be able to cut it, even with carbide. And remember, you still have to ream the chamber, so a hard barrel shank is not a good thing.
If you make an undersized bushing and press it on, it will get driven off by the pressure of firing. If you are going to turn the shank down and put a threaded bushing onto it, thread the OD of the barrel shank and the ID of the bushing, then LocTite it in place. I have done this in the past, it works well.
But all this is going to be a lot of work on a pre-fit barrel, and it will be ruined if you goof it up. I would still get the contoured barrel blank and thread it, and sell the barrel you have in hand. But it's your project.