45/70 for black bears
I can't speak to the effectiveness of a 45/70 on grizzlies but I can tell you from experience that a black bear is a relatively easy kill. I have taken two bears with a 50# recurve bow at 18 yards. If you're planning a spot and stalk hunt and expect to shoot one at some distance, I would think a 30.06 is plenty for the job. I have a friend two dropped a 275# boar at just under 100yds with a 30.30. I'm not necessarily recommending that.