FT45
I see this topic come up every year or so and to be perfectly honest I cringe when I see it. You will gain very little by doing this rechambering unless you intend to shoot black powder. The limiting factor here is the strength of the action and a 45-70 loaded with smokeless can easily exceed the strength of all but a few suitable bolt and single actions. By rechambering to a longer case all you are gaining is air space, which you will probably have to take up with a filler, adding another variable to the equation. Now toss in an expensive set of dies and hard to find cases and the level of complexity has gone way up. As has been mentioned the three levels of 45-70 loads will cover anything in the Americas and some will say the world. Action strength will limit you to the lower and possibly the mid-level loads. I shoot all three in 4 rifles and have shot some very hot loads in my Ruger #1, ONCE! After you regain your bearings and recalibrate you eye balls, you'll be happy with the lesser loads.