What I have learned about molds and especially molds for a military rifle in 58 caliber, is to wait until I have the rifle in hand and the bore measured. Now this will be a bitt difficult on your 1861 Springfield since the Pedersoli rifle has three lands so measuring is a bit complicated. Pedersoli recommends a 0.577" mold, so in my opinion,
@bisleyjohn's suggestion would result in a Minié ball too small of a diameter to provide accurate on target performance. I suggest an inexpensive digital caliper to measure the diameter of the barrel from the top of a land to the bottom of a groove. Since you have a round barrel, measure from the bottom of the groove to the outside of the barrel. Measure from the top of the land to the outside of the barrel. Subtract the groove measurement from the land measurement to get the groove depth. Subtract the groove depth from the land to groove dimension to determine the land to land diameter. Use that dimension to select a mold and perhaps a sizing die.
Also interaction is not determined just by the particular Minié ball dropped from the mold, but includes the lubrication applied to the grooves as well. The tallow, tallow and bee's wax, oils and bee's wax and a few other somewhat greasy lubes can work well. Plenty of suggestions exist on the Forum. You just need to determine what works best for your rifle, Minié ball and lube.