Unless the rifle was rebarrelled it is a .56-56. That cartridge used a heel bullet, where the outside diameter of the bullet ahead of the case mouth was the same as the case diameter at the mouth, and the portion inside the case was smaller. There are several choices in reloading for this rifle:
The designation of ".56-.56" or as usually printed, "56-56" refers to the nominal outside diameter of the cartridge case. In reality, the groove diameter of the barrel was around .54 caliber. The original terminology for the cartridge was "#56 cartridge".
First is to determine the actual groove diameter of the barrel. This can be a bit tricky. I have never measured a rifle...only M1860 Spencer Carbines. The interesting thing about the carbines is that the groove diameter at the breech, just ahead of the chamber is .545". But the rifling is tapered and measures about .535" at the muzzle of the 22-inch barrels!
One would be to find a bullet or bullet mould for a heel bullet. I am not sure where you might find one, but possibly could have one made for you. In that case, you would have to determine the inside diameter of the shortened .50-70 cases, as the walls will be thicker at the mouth with the shortened case. Then, depending on the bullet alloy you use and the amount of expansion of the bullet after casting, have the final diameter of the heel about .002-.003" larger than the inside of the case mouth.
Another possibility is to use a straight-sided bullet and ream the inside of the brass around .003" smaller than the bullet diameter. I had good luck using bullets cast from #2 Lyman equivalent alloy (9lbs wheelweights and 1 lb. 50/50 bar solder) and smokeless powder. I did not do much with black powder. ONE WORD OF CAUTION: WHATEVER BULLET YOU USE, IT MUST HAVE A MEPLAT (NOSE) AT LEAST .250" IN DIAMETER! AND PRIMERS MUST BE SEATED FLUSH OR BELOW THE SURFACE OF THE CASE HEAD! This is to prevent a magazine explosion that was less of a problem (though not completely) with the original rimfire cartridges! Such an explosion is bad enough in Henry repeating rifles, as the "only" thing threatened might be your hand/arm. But a magazine explosion in a Spencer, with the magazine tube in the buttstock could ruin your whole day!
Most of the bullets I have used in .56-56 Spencers were cast from a variety of moulds made by Rapine, who is unfortunately out of business. Another, mould, also no longer available is a Lyman 533476AX hollowbase mould, casting a bullet around 412 grains with the Lyman #2 alloy. These were sized to .535- .540" depending on what worked best.
I also found it best to anneal the cut-down .50-70 brass about half-way down the remaining case. I place the brass in water about half-way from the bottom of the case, heat with a propane torch, rotating the case until it discolors and then tipping the case over into the water.
I know this is a bunch of work, but once you get things sorted out, it can be a lot of fun. Hope this is of some help. If you would like more specific details about loading for this cartridge, let me know.
Stay well and safe!