Like ArchAngelCD said IMR4198 is a very good powder for the 45-70. That is what I used from 1984 till last week. I tried 3 different loads of IMR3031 under Hornady's 350 grain round nose bullet, and FOR MY RIFLE one of them shoots a lot better than my old IMR4198 load. I will load up about 15 more shells with the same IMR3031 loading and see if it is repeatable.Three more 5 round groups will make me fell better about the grain charge that looks very promising. I know that four 5 round groups will not be proof positive I found my best load, but more groups over time will tell. It is fun trying to prove that just one good group is repeatable. It is fitting that a powder like IMR3031 that has been around since WWI, will give new accuracy to an even older cartridge design.
It is nice to see that the 45-70 is so well liked and reloaded. I felt like I was alone in the wilderness when I had mine semi-custom made in 1984. Fun gun, and has been easy on brass, even with full house loads. They just have to be trimmed more.