I certainly wouldn't do anything to dissuade you from purchasing a 45/70. I own one. BUT, it sounds as though your needs would be met with a .44 mag, or, if you handload, a 45Colt levergun.
In handguns, those two calibers have taken most everything that walks or crawls, so it follows that in a levergun they'd do a little more. The levergun should give you +400fps over the published h'gun velocities.
Check out what these guys do with those calibers and you'll get some idea how these two calibers stack up to the 45/70.
http://www.cor-bon.com/http://www.buffalobore.com/http://www.garrettcartridges.com/Take care, have fun, and buy one of each.
John
