Rufous,
My go to load in my Inline Black powder rifle, was 300gr hornady XTP over top 90gr of 777. That produced around 1600 fps. I could fill a dump truck with the deer I have knocked over with that load. I have NO doubts that the 325gr out of the 45-70 at the velocity you stated will serve you well. All of my shooting was done inside 150 yards with the large majority being under 100 yards. I know this is not an apples to apple comparison, but keep the ranges sane and you should be fine.
