Re: Barrel swap
Which Mauser action do you have?
If the barrel that you are looking at is not a custom barrel, I would simply check around for a new take off bbl for a Ruger in the chambering you want, minor fitting required.
If it is a good custom barrel, I would cut off the bbl tennon, rethread and rechamber anyway. Headspace will not be correct the way it is, and chambering will more than likely be required anyway, and you'd still end up with a half ass thread job cutting the new thread over another... if even feasable to begin with... I just wouldn't go there if it were me.