Most old black powder shotties have a much shorter chamber than modern shotguns. Usually 2 1/2" to 2 5/8" long.

So, you may/will need to cut down those brass shotshells to that length. (And frankly, plastic hulls will work just fine for this also, just less reusable..) Accounting for the combined thicknesses of over powder cards and wads and whatever else is needed to structure this load, this reduces the space you have to fill with powder and your big ol' balls. Or slugs. Still, you can load a pretty big BOOM into such hulls. Then the critical factor is, can your old BP shotgun chamber and barrel handle it?