the 168's from a 20" tube will get you to about 750 yards. We've had guys shooting 20" .308's at our school and at matches we host, and that's about where the 168's from a 20" barrel will get to before they get squirrely.
Federal's 175 grain Matchking loads are a good option for taking the rifle all the way to 1000 yards.
Where are you planning to shoot? Reason I ask is we use 24" square plates at 1000...