Any bullet will fit, some just have to be loaded to a shorter COL in most 45-70s due to their short throat which is common. You can see the difference in bullet ogives that makes some hard to load to the COL listed in most data and the reason many of us here have throated our 45-70s. The ogive shape of the bullet determines how long a cartridge can be loaded relative to the throat depth, that's the problem with the 405gr Rem and 400gr Speer and even most 350gr, their fat ogive makes a big difference compared to the smaller 300gr bullets. You can see the difference in the comparison below, L to R, 300gr Speer, 300gr Rem, 405gr Rem, 400gr Speer FN, 350gr Speer FN, 350gr Hornady RN and 350gr Hornady FP Interlock. The 350gr Speer would be a likely bigger bullet as it has a better ogive shape.
Tim