I used a 338 win mag, I had planed on using the 45-70 but my son had left the shells in a storage shed that I could not get to late at night.
I shot him no less than 3 times in the neck, My final shot was through both lungs, when we gutted him there was No blood in the lungs. I found only minor damage to the spine. I do not blame the bullet for the slow kill it was poor shot placement, 2 rounds went just above the spine 1 was just under. a 4th round may have been in this area but there was a lot of tissue damage and the hide was cut there.
I did get 360lbs of burger and around 80 lbs of steak and roast.
http://img.photobucket.com/albums/v650/ldjanos/My%20Bison/Bisondad006.jpg