Does the 300 Win Mag really offer that much over the 30-06 that it's worth the extra cost? Same bullets are used, same COL, 30-06 is a proven long ranger perform, as is the 300 Win Mag. 30-06 has less recoil than 300 WM. with that said, wouldn't the Accuracy International stocks for Long Action Remington 700's work for 30-06? AI Mags for 300 Win Mag should work for 30-06 as well.... In theory since COL is the same or is the 300 WM case that much bigger that it wouldn't work?
The 300 Win Mag has about 200 more feet per second over the 30.06. For many this won't matter, but when you start shooting the distance, it does. There can be many other factors in the picture, but at 1000 yards, the 30.06 can easily drop another 10 inches over the 300. 30.06 is also more susceptible to wind.
If one is shooting under 500 yards, I'd go with the 308 or 30.06. Going over 1000 yards, 300 Win Mag or larger.