One thing about Mag-Na-Porting. Do not believe the story that it will reduce recoil by 30%. It will not. I have a .458 Win mag and a Marlin 1895SS, both Mag-Na-Ported, and it does not reduce straight back recoil a bit. What it does do is almost eliminate muzzle jump, and if that muzzle jump is busting your chops, you MIGHT conclude that it reduces recoil albeit falsely. The straight back recoil of these two rifles was not diminished one bit that my shoulder could tell, but my cheek, especially on the 1895SS, could tell a world of difference. For the elimination of muzzle jump MNPing is worth every penny, but if your shoulder hurts and is getting bruised by straight back recoil, do not expect MNPing to do anything to eliminate that.
To the OP, I have quite a few 45-70s from single shots to handguns and a couple of stops in between.
If you want the without a question the strongest, Ruger No. 1 hands down. If you want one that is strong enough for all but the most insane loads (and in the 7.25 pound No. 1, heavy loads take you out for a heavy beating) and more than strong enough for anything walking North America, the 1895 is the one to get, in any form you want.
By the way, the No. 1 is 7.25 pounds, not 9 plus. My load is a 450 gr. bullet at 1825 fps with a 53.5 gr. load of H335. This combination in the No. 1 generates just a bit more recoil than listed above. 43 ft-lbs and 20 fps recoil velocity. That will get your attention. If you get the No. 1 and want to go heavy loads, the first thing you do is come home, remove the factory recoil plate (calling it a pad is a joke) and replace it with a Decelerator. Your shoulder will thank you.