I go with Pete's recommendation. The Gould bullet as made by Lyman has been the "standard" for the 45/70 for "light big game" for about a hundred years.
For really large game, there is nothing at all wrong with the 500 grain Gov't bullet. I have had luck casting and loading this for Bison for friends. The most important thing with ANY cast bullet is to use a SOFT alloy. Anywhere from 50/1-30/1 lead and tin. On Buffalo, the Gov't bullet will expand properly with the right alloy and still give the necessary penetration. I would stay away from any alloy using antimony. I had a friend shoot a deer several times chasing it over a couple of miles of deep snow in Colorado with hard bullets and it made a believer out of him (he wouldn't listen to me
and thought that wheel weights would do just fine. He learned and of course, I also learned. The loads did not expand and just penciled thru. This was a mature Mule Deer so it wasn't exactly tiny. My friend is an excellent shot and quite "woods wise". He just had the wrong alloy.
So, to reiterate, for light big game, the Gould bullet is premier, the 400 grain flat nose is "ok" and for really large big game (Elk, Moose, and Bison) I'd go with a 500+ grain bullet,
The alloy recommendation is the same - 50/1 to as hard as 30/1 (I lean towards the soft end). This alloy is mallable and will expand readily and hold together. You should not have any leading problems. I would use Emmert's home mix, SPG, or Pioneer Products lube. You can use an Alox based lube with smokeless but I would, no doubt, go with black powder and the above lubes.
Good luck!
Dale