I'm far from an expert...but for the heavier bullets..I've seen a 1:8 twist used. I've got a 1:8.5" twist on my .260 Improved. I based this twist rate on the article: "Terry's Tactical Two-Sixty AI" http://www.6mmbr.com/gunweek046.html

I'm sure others with a ton of experience will be able to give better insight and advice.

Good Luck...Sounds like an exciting project in the works!

Wayne