400 grain arrow is just fine for 68 lbs. For sighting in here's what I'd do.

Sight in your top pin for 20 yards.Then walk back to 30 yards. Using your 20 yard pin at 30 yards, shoot 4 arrows at a 1" dot. They should hit4 to 6" below the dot depending on your arrow speed.

Now,adjust your 30 yard pinso that when you are at full drawthe 30 yard pin is in the middle of the group of arrows that you just shot at 30 yards. Now your 30 yard pin should be pretty close. Fine tune from there.

Once you have your 30 yard pin dead on at 30 yards, repeat the same process for your 40 yard pin. Shoot the 30 yard pin at 40 yards, (they'll probably be 10-12" low) and then put the 40 yard pin in the center of that group of arrows while at full draw. This will make sure that if you won't be slinging arrows over the top of the target.