If you are good enough a shooter there are several documented and laser ranged one shot kills on bison out to and beyond 1000 yards with the 45-70. Granted these were not with an off the shelf lever gun but the cartridge is certainly up to the task.

As for elk, just like everything else regarding effective ranges,it will depend on how much you are willing to practice and how much time and money you are willing to spend finding the perfect load for your gun. For a factory gun with factory ammo 200 - 300yards is a piece of cake for the cartridge.