I ended up in the same situation as you about 15 years ago. First tried finding a non-magnum bolt but the search was fruitless. Ended up having my gunsmith re-chamber to .30x.350 Rem Mag (cheaper than a new barrel). Since my little thumper has a hair more case capacity than a .30-06, that is what I use for load data. That little sucker roars like a lion, breaths like a dragon, and sure is a lot of fun to shoot. I have 4 M600s, a 6.5 mag, a .308, a .30x.350 Rem mag and a .350 Rem mag. If you have not shot a M600 in .350 mag with 250 gr bullets before, be prepared see God. I ended up installing a receiver sight on my .350 because of scope cuts. Think twice before going all the way to .350 Rem Mag.
Good luck and please let me know what you end up with.
George