With the .45/70- it all boils down to how much pressure will the gun handle for an everyday appetite.

As to modern-type loads - 34,000PSI will get you 2,000fps out of a 24" barrel with 400gr. bullets with only a couple powders, and 2,000fps to 2,100fps with 350's.
43,000PSI will get over 2,000fps with more powders, as well as run over 2,200fps with 350's and there are some excellent 350gr.'ers out there. What do you think you need? As 88 Mau. noted, a black powder load, placed in the right spot dumped a previously wounded buffalo. Smokeless powder loads with descent bullets, can pack a tremendous blow and take out big brown bears with ease.

Anyone who would shoot a buffalo with a 300gr. Factory .45/70 deer load should be whipped. - in my MOST humble opinion, of course.