I THINK if

You took the old barrel off and sent it to whoever is making the new barrel, you could do the rest of the work yourself. It seems far less complicated than a bolt action and trying to get a tight head space.

I also think GM still makes a fast twist 45 barrel in 1-18 with cut rifling. It would need to be contoured unless you want a REALLY big barrel.

It sounds like Douglas can do almost all the work for you. Not sure about a breach plug though. http://www.douglasbarrels.net/pricelist/index.html

Turned, Threaded Cut to Length Crowned & Polished (FTC) $323.00