My understanding is that it is SASS lrgal to use an original 56-50 converted, just as it is SASS legal to use an original 1870s vintage .45 Colt SAA with a .454 bore and tin rimmed original cqases instead of a modern clone with a .452 bore and large rimmed modern cases. They original uses essentially the same brass as the Armi Sport, with the exception of slightly larger rim than the clone. The bore of an original 1865 model varies from .515 to .520, were the Armi Sport shoots a .512 to .515 bullet. Identicle powder charges, too.
If you get any grief from the RO, you might use my annology. I used it to shoot a .56-56 1860 model as a main match gun, citing same gun, variation on caliber.