3x9 is a little on the light side for 600 yards. I would not use less than 24X and prefer a setting of 36 for 600 yards on my 42 or 55 power Nightforce Scopes.
For a .308, I think you will need a rail with some elevation built in. If you are also going to try shooting at 25-50 yards, that will be you next challenge with the rail that has elevation. There are plenty of free ballistics calculators on the iPhone App list or on the web using JBM. You will be surprised at how much hold over you will need for 600 yards sighted in at 50.
One option is a set of Burris Signature Z rings. They can be bought with a set of inserts that allow a lot of flexibility in how much MOA hold over will be built in. They also eliminate ring marks on your scope.
Make sure they are high enough for your barrel contour so that you can tilt the scope down to get the barrel up and lob your bullets in at 600.
On my 40X in 6mmBR Norma, I have one scope sighted in for 200 yards and shoot 100 with Hold Under and 300 with Hold Over and then a second scope with a set of 25 MOA Kelbly Rings, that I switch when I want to shoot it at 800 yards, which it is set for and hold under for 600 and hold over for 1,000.
A compromise is not always the best, but it depends on what your end game is.
Even with holding over 38 MOA I have shot groups under 8 inches at 1,000, but under 6 inches with the correct scope set up on the rifle and under 4.5 inches with the correct scope sighted in at 1,000 so for me at least, it does make a difference.