A .457 roundball should be a tight fit in a sized .45/70 case, no need to crimp, especially since it's a single shot. With a full load of BP, at the velocity it's going to be moving, and the small bearing surface of a roundball, it might strip out of the fast twist rifling of a Sharps. Remember, roundballs were designed to shoot out of a slow twist muzzleloader. If you could use a lesser charge of powder with a filler, like Oyeboten recommended, you might get better results. A tight fitting lubricated cardboard wad behind it, like used for loading cap and ball pistols, might be helpful if you decide to go with a full load. If you don't have your BP yet, you might try a low velocity powder like IMR Trailboss. If you are not familiar with this powder, it is used by Cowboy Action Shooters as a low pressure smokeless powder in rounds originally designed for BP. 14 grains will almost come up to the bottom of a roundball in a .45/70 case. No need to use a filler with this powder, either, and no fouling.