The SB2 frame on a late madel 45-70 will handle loads up into the level 3 ruger loads, but most shooters find the recoil too much unless other steps are taken to reduce the felt recoil, like adding weight to the stock bolt hole and/or upgrading the recoil pad. As has been said, if range is held to 150 yds or less, even the trapdoor levels are sufficient for taking most north american game. If you want to extend the range over 200yds, heavier charges are needed to keep trajectory flat which can be punishing on both ends!! My 2400+fps 300gr nosler partition load has a MPBR of 242yds with an 8" kill zone, which is fine for elk in the areas I hunt.
Tim