First I will discuss old eyes and iron sights. Receiver sights are a good way to go as they both increase radius and offer better focus. For your type of shooting I would also look at a brass bead no smaller than 3/32. The front sight is 1/2 the sighting system. For regular iron sights another very good option is to switch to an "Express" sight. I did so on my flintlocks as receiver sights don't fit. An express sight does not have a notch to settle the bead into but is a shallow V and you settle the bead into the bottom of the V but see the whole V. They got their name from use on the big double express rifles used in Africa for quick shooting. Another option is to get a round needle file and open up the U in the rear sight so you get more light around the bead. You will also need some touch up bluing.
As to the 45-70. I traded off my 45-70 for a 38-55 and never looked back. Its more than needed for anything I will hunt.
DEP