I was always told the most belted magnums space off the belt. The biggest problem you'll run into with a rechamber of like cartridges is if one has more/less taper than the other. If so, the reamer won't clean up the old chamber and you'll have a chamber where the old shoulder is still existing while the new, deeper chamber runs past it, causing a false shoulder along with the new chamber. Pretty obvious the problems this can cause when the case expands after firing.
Haven't looked at the SAAMI drawings for these cartridges to see if this would be the case here, but as I said before to the OP, it would only take 5 minutes to do a search and find out.
I'm not sure I would be too impresssed with a smith who didn't remove the barrel for this procedure unless he had lots of experience and could convince me it would be done properly or he would fix it, quickly.
Bob
