Well. No matter what you use, bullet placement is the key. I have a copy of Clyde Ormond's work, "Bear". He has a story in it of an Alaskan resident who killed a brown bear with a 22 lr. Yes. That is it, a 22 lr. The trapper was in a boat and shot 2 times between the ribs into the heart. He rowed out to deeper water and the bear eventually dropped dead.
This is an extreme example, but it shows you that bullet placement is the key. The 45-70 Contender has been used for all kinds of game including Africa's dangerous game. I would say it would work well, provided you hit the animal in the right spot.