44-77
According to articles by Mike Venturino, until the 45-70 was introduced, the 44-77 was the most popular caliber chambered in Sharps rifles, but it was a Remington developed cartridge and was first introduced in Remington rollers, and the Sharps New Model 1869. It was used extensively by buffalo hunters, but was also chambered in Remington and Sharps Creedmore rifles.
He notes that bore diameters in original rifles vary considerably, and says that he personally has slugged barrels varying from .446" up to .457".
Lyman makes #446110, a nominal 340gr, .43 Spanish mould at .446", RCBS make a 44-370FN which supposedly drops out around 400gr., and I'm sure you already have .457" moulds for your other 45-70 roller, so bullet supply shouldn't be a problem.
It has brass, but if needed, Bertram make it, and that and re-formed brass is available from Buffalo.
If the price was right, I'd say go for it. Just think of the history that rifle has.
Regards, DaveF.