At one in 36 inches for a .58 caliber bore, this rifle was almost certainly designed for a bullet rather than a round ball. You might try something that casts out at about 450 grains to start, although it's possible the original bullet could have been lighter if it were a hollow point. I have no idea what an appropriate powder charge might be, but you wouldn't go far wrong by starting with a powder charge of 55-60 grains of FFG powder and working up from there until the barrels shoot to the same point of aim.

Let us know how you make out.