Avanger,
Little question that there are better powders for the 45/70 then 4350. I use H335 in mine behind a 465gr Wide Flat Nose cast. AWESOME.
But since you are specifying 4350, you might look up information for the Accurate Arms 4350, they list loads with their powder from 378gr up through 500gr.
Crusty Deary Ol'Coot