With 300 grain bullets (Sierra, Hornady and Speer), 54 grains of IMR3031 is about a 28,000 PSI load in the 45-70 according to IMR. 59 grains is a maximum load in the Marlin 1895 according to Speer. Winchester brass.
With 300 grain bullets (Sierra, Hornady and Speer), 61 grains of IMR4064 is about a 28,000 PSI load in the 45-70 according to Speer. 63 grains compressed is about all you can get in a Winchester 45-70 case which would be maximum for the Marlin 1895.