If you are dead set on the Wby then go for it, but the 270 WSM is easily the equal, and the better in terms of efficiency, accuracy and overall utility.
My buddies that have befriended the 270 WSM see all the range and no difference on game waaayy out there to 1000yds when compared to their 300 Win rifles, and with half the recoil.
That's their experience but they have a lot of that and basically know what they are doing. If the Wby was better in any way their rifles would be so chambered...a chamber is just a hole in the barrel.
Both chambers will need a rebarrel to a faster twist that can make the best use of newer, high BC bullets for the caliber if the goal is longer range. If long range isn't the goal, then there is no point over the 270 Win.
Either way, I'm not really a 270 guy, so if I were going to rebarrel a Montana I would go 7WSM or a similar flavor.
Bullets make a difference...chambers not so much.
