7MM Remington Magnum.
But since you already have a .308 the Caliber maters less than the shooter.
If you can hit a pie plate at 400 yards 80% of the time use your .308 with 180 Grain Bullets.
If not practice more with your .308. Even a .375 H&H Magnum doesn't kill an elk much quicker than a good hit from a .30-30 Winchester closer than 100 Yards.