A former acquaintance used a 6BR with 65gr VMax for PD hunting and I believe his was a 13 or 14 twist. With your home elevation and the added velocity of the 6AI I would bet you could go a bit heavier. Of course length will be the deciding factor. I found an article on 6BR that showed some success with with the 75gr VMax at 3800fps in a 6mm AI.
http://www.6mmbr.com/gunweek042.html
