I have hunted with, and shot the .45/70 in competition for many moons......most all of it with cast bullets & black powder. However, I have no experience in shooting brown bears with one, but I do know that a 500 gr. cast lead bullet/blackpowder load will pass completely through a buffalo LENGTHWISE. You can't put a bullet of that length in your Marlin, but any well-constructed bullet of 400 gr. should work fine. Why don't you consult somebody that you trust that sells reloading components up there? They should know what works best for what you are trying to do.