Howdy
I agree with several posters. The 38-55 of 45-70 is all you need. The bigger more powerful versions of the 45-70, such as 45-90, 45-110, 45-120 are over kill, besides, it is much easier to find brass for 45-70 than any of the larger cartridges. Loading Black Powder single shot cartridges takes some trial and error. There is much, much more information readily available about 45-70 than any of its larger cousins. This is also true about any of the other less common single shot rounds like 45-65 or 50-70.
In his book, Shooting Buffalo Rifles of the Old West, Mike Venturino emphatically states to try the 45-70 first before any of the other more exotic cartridges.
You might try reading his book. Currently out of stock at Amazon, but not too hard to find if you look around. He describes all the old single shot rifles, all the old cartridges, and how to load all of them with Black Powder. Best book on the subject.