Here is an interesting chart and explanation of cup and psi ,I think this is the article by bramwell mentioned in the link daryl gave above . also keep in mind that even if you had a rifle with a strong action some of the 45-70 cases might not hold up to higher pressures being as they were originaly designed for low pressure .I've heard some of the newer 45-70 cases might be a little thicker.
http://www.shootingsoftware.com/ftp/psicuparticle2.pdf