Stat - Here is the data right out of my old Lyman Reloading Manual for these bullets with your chosen powder of 3031. These loads are for modern 45/70s ONLY.

300 gr cast bullet - 3031 - Starting load = 44 grs - Velocity = 1517 fps
Maximum load/3031- 50 grs - Velocity 1782

400 gr cast bullet - 3031 - Starting load = 42 grs - Velocity = 1492 fps
Maximum load/3031 - 48 grs - Velocity = 1745 fps


Hope these help you.