SHR 970 is spot on with his comments. The gun is rated at a certain pressure level. In the case of the 45-70, data is supplied for the specific type of gun. Why go beyond what others more qualified than us deem unsafe for the gun?
I shoot the 1895 Guide Gun. 3031 has the potential to go beyond safe loads in that cartridge. Varget is a great powder for the 1895 and you can't put too much powder in the case. H-4895 steps it up a notch but still safe. I get around 1960 fps using H-4895 and 350 grain bullets. I did not find 3031 to be as accurate as the other two powders. Reloader #7 is also a good powder for the 45-70 but has a noticibly sharper raport than the Hodgdon powders.
Gun control is hitting what you aim at..