The .577-450 Snider military round was loaded with a paper sleeve in the case to reduce its capacity appropriately. Your cases seem to have a similar sleeve, albeit not paper. My inexpert guess would be that it would be good to measure the volume of your cases and duplicate that volume when you choose the weight of metal to make your new cases. The appropriate powder volume would be that required to fill the case below the ball.

I would also think a round ball would not require a wad. The very shape of a ball spreads the heat over a larger surface than the base of a conical bullet, thus avoiding the melting/vaporization that causes leading. Recall that muzzle loading rifles use only a cloth patch with a round ball, sufficient to engage the rifling but of little use in protection from heat.

What a wonderful gun! You are very fortunate.

P.S. Graving rifling with a bullet, especially one lightly crimped, is usually a recipe for a bullet left in the bore if the unfired case is extracted. I'd back off a little.

Modern rifles usually like a little jump, but not a lot, but it can vary from rifle to rifle (according to Don Zutz). I'd leave about .005" leade and see how it shoots. BTW you are also probably in for a lot of fun trying brands of BP to see what the rifle likes. Whatever it was regulated with is certainly no longer available.