Well,,, Ruger offers a "Single-Seven" in 327 Mag as you have mentioned. I have not seen any take off barrels,,, but I'd say try & talk to custom gunsmiths to see if they may have a spare barrel. Any good gunsmith should be able to install it.
Try our own member 2 dogs to start with. (He's been fooling with a few customs,,, and may have a barrel.)
After that,, maybe Dave Clements, Bowen, Hamilton, etc to see if a barrel is to be found.