Why would you want to shoot a conical in what is one of the best muzzleloading North American hunting rifles? You will gain nothing in using a slug - except instabiltiy upon impact & an incredibly high trajectory due to a loafing, heavy bullet. That rifle has a RB twist. It is not a 12 bore Double English rifle for dangeorus game and will not take more than squib loads without prematurely burning out nipples. Too - the nipple/hammer/fence relationship is lousy and not safe for heavy pressure loads - which, slugs deliver in spades.
The proper round ball for that rifle is a .715", which in pure lead weighs 545gr. (1-1/4oz) and 495gr. or a bit more in WW metal. Crown the muzzles properly and learn to shoot patched round balls.

Just my opinion, of course. It's your rifle adn your eyeballs without any protection from cap fragments. As I said, it is an unsafe breech due to the fit. Pedersoli put potentially great fences on the rifle, then used his junk, el-cheapo locks,mounted the nipples above the fences and made them inefectual. Bloody hell.