k80
What do you want the rifle for?
For North America the 45/70 loaded hot is plenty enough for any game suitable for a single shot.

If you are talking about hunting in Africa either the 450 3 1/4" or the 450 No2 is a good choice, but recoil wil be stout in the Ruger No1.

It would be simple and in-expensive to to rechamber for the 45/120. Dougaboy has done this and tell you the Ballistics.
I would think in the No1 you could get close to the 458 Win Mag.

If I was not planning on using it in Africa I would leave it as a 45/70 and just load it to Ruger 45/70 pressures.
I have a standard Ruger No1 in 47/70 and my standard load has all the recoil and power for ANY North American game.