The ask was a caliber for big game and yup 45/70 is not a 400-yard shooter but typically good up to 200 yards that said it will take down anything in North America. I took down a Kodiak with a 45/70 in 94.The thing I'd be concerned about with 45-70 is distance - I certainly don't know from firsthand experience, but I've read that sometimes getting a close shot at Elk in the Rockies just doesn't happen, so you need a rifle that can comfortably reach out to 400 yards or more. I'm not sure I'd want to think about those trajectories with something like the 45-70.
I have a 35 Whelen - it can push a 225 gr bullet almost 2700 fps, but that's another one that starts to drop hard past 400 yards.
For comparison sake, the 35 Whelen has a drop between 16 and 20 inches at 300 yards, depending on the bullet and velocity. By comparison, the 300 Win Mag has a drop of about 6.5" with a 180 gr at 300 yards.
To keep things simple, I don't see any reason why a 300 Win Mag wouldn't get the job done nicely and with a plethora of factory hunting loads to choose from.