A member here uses far less than 45/70's on buffalo and receives only praise. I say go for it.

On the subject of lever actions, anyone see the 50 Alaskan elephant?
A modern 1886 can throw a 525gr (.50-110) at 2100fps..That would be fine for anything as far as i'm concerned. (no real world experience - i've never seen any of these animals in the wild)