Hi Civil War Life,
Congratulations on being the owner of an 1866.
I would like to suggest to you that you run a chamber casting on your rifle before purchasing .50-70 brass. The reason I say this is, I have read stories of people owning 1866s that have a chamber longer than standard length brass. I have not heard of this condition in 1868 or 1870 model Springfields. It is just a suggestion, and once you have determined your exact chamber length you will be able to trim your cases to match from .50-90 or .50 basic brass. It will help accuracy considerably. At the same time, slug your bore to determine your exact bullet dimensions.
I believe the Lyman 515141 will be closest to the original Arsenal type bullet, but I am not sure of the diameter it will drop from the mould with a soft lead alloy, which is what the rifle was originally intended to fire. Stay away from alloys that contain antimony, as the bullet will be too hard. They have to "slug up" in the bore upon ignition.
Custom moulds can be had to fit your bore, but can be expensive. But in the long run, it will be well worth it.
Good luck.