Power isn't a question with the 45/70 as it still has more punch at 400 yards than a 44 mag does at the muzzle. The only issue is the trajectory, at that range you're looking at a 95 inch drop. Nobody has any buisness holding 8 feet over the target. The 200 yard drop is much better, only 12 inches.
If you do plan on the possibility of those long shots then consider the old ladder sights. They work much better than you think. A lot of practice helps too.
I've taken a 250 yard shot with a 30-30 and killed the deer with one hit. It was a 14 inch drop but I practice a lot so I knew I could do it. Having a good laser rangefinder helps too.