schmel, I can't see anyone using the .45-70 for ranges that far. You know what kind of bullet drop you're getting at 500 yards, much less 600 or 1000? That one link above has it dropping 300 inches at 500 yards! At that trajectory it would be coming almost straight down. Not to mention the fact that the amount of energy it has that far out would not do jack to American Buffalo.
In 1873 the Army adopted the .45-70 as it's standard rifle cartridge. Sharps was the main supplier, especially for the cavalry soldiers.
About this time, Creedmore 1000 yard shooting competitions were extremely popular and Sharps Rifles made many variants of target rifles specifically for this competition. Most were in the .45-70 chambering, because that was military standard. They also made custom cartridges such as the .45-90, .45-100, .45-110, .45-120, .50-90 and .50-120 as well. In their heyday, Sharps was probably the highest quality and highest accuracy rifle you could get.
Most buffalo shots were made in the 200 - 400 yard range, if I'm remembering some obscure reading correctly. Not a short distance, and not nececarrily a long distance either. But there were recorded cases of extremely long kill shots as well, much like there are today. In fact, I read about a shot in an indian war of about 1 mile that hit it's target - didn't kill the indian, but it still had enough punch at that range to knock him off his horse and make the rest of the war party think twice about continuing their attack (I believe this was during the Adobe Walls raid on the Canadian River in north Texas, actually). It was not common place by any stretch, but it seems to me that you are shortchanging the capability of the cartridge by stating that it's a 200 yard or under round, when it is not.
I also likened shooting something along the lines of a .45-70 to lobbing artillery in - you are using high angle trajectories to get the projectile that far, but the mass of the round is what makes it deadly.
Remember, 405 grains (standard projectile weight of the time period) has a lot of mass. It doesn't have to have blazing speed to do some pretty good damage.
Is it something that I'm recommending to take to a 1000 yard shoot? Not even close, but it has far more capability than 200 yards, given the right platform.
I wouldn't shoot my 1895 past about 200, and I'd probably be lucky to hit something at that distance. Mostly because the recoil is brutal, and that would be a hell of a good shot with open sights. But a Creedmore rifle with a 34" barrel and a telescopic or vernier sight is far more than capable of putting accurate rounds considerably further downrange, which goes back to my mentioning what type of platform you are choosing.