Your upcoming hunt sounds very exciting!! I have seen a couple Television programs of people hunting Bison. And while yes, they are a herd animal, and if you can get close to the herd they should be fairly easy to shoot, the shows I watched.. the problem was getting close to them. It seems if the herd has been hunted they get cautious of hunters. Which is great. Many people picture modern day Bison hunts as walking up to a fenced in paddock and simply picking out the animal they want and shooting it and paying for the privilege of shooting one. Like I said, a free chase hunt sounds exciting.
As for whether your load will work, I can not say. I know what I would be shooting if I were hunting, but that does you little good. All I can say is if the load is accurate and you keep your shot range with in your ability, you should be successful. Whether you really need a hard cast bullet I can not say. As penetration of the animal to the vital organs is what your after. Then you want expansion of the bullet. Think in terms of what weapons and bullets were used back in our history to harvest Bison and you load sounds pretty good.
Keep us posted on your hunt. And when you do.. remember, we love pictures!!!