I have used the search function, and I have found the following:
-There were some machinegun barrels that were adapted some time ago, now unobtainium.
-Swapping Mosin barrels will require the extractor relief to be cut, not and easy task and requires good machine tools available.
-Someone was told by an unnamed barrel manufacturer that they wouldn't do it.
-An (possibly) intelligent approach would be to use a more standard barrel diameter to shoot more easily available projectiles(.308, etc) and size the brass a little tighter to grip the slightly smaller projectile.
-This process is likely to cost 5-6 times what I paid for the gun, minimum. Many people discourage any serious work on a Mosin for this reason.
Google has provided a list of barrel manufacturers, so far none have answered my inquiries. None list 7.62x54r on their websites.
I have found a wealth of information related to the question that I asked, but I haven't found anything that answers the question that I asked, a manufacturer willing to do this kind of work. If I have overlooked a thread that should have answered this question, please point me in the right direction, but I did try searching before I posted.