I'd never shoot lead #6 shot at a distance beyond 40. That's pushing your luck. Personally, try to keep all your shots under 40. If you feel compelled to take the 40+ yarder, I'd multi-pattern with the #5 shot, (or better yet, Hevi-shot), a good number of individual targets to establish an average pellet count in the spine and brain.

If you gun will not consistently and without fail place 6 or more pellets of the #5 shot in the brain and spine, don't take the shot.