I'd go with the 375. I think you'll find terminal performance for a .375 with a 270 gr x or similar bullet will give you all the penetration you need, and I'd expect a larger dia wound channel than the 45-70. I've found a bolt gun can provide quick follow up shots, I've never timed one vs. a lever but I think you'll find not much difference. While the trajectory of the rounds doesn't matter for close in work, the 45-70 is barely a 200yd round w/o compensating for drop, the .375 w/ 270's is an honest 300 yd round. Finally I'd say day in day out dealing with water and grit the Ruger ss 77's are one of the toughest guns made.
