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.
That's exactly what I did -- individual targets each and every shot. Consistently put about 12 in the brain and spine at 50 yards. The 3 1/2" #5's are what I'm planning on using, but with all the good news stories about the Hevi-shot, I might just have to try a box of them!