Since we are talking black powder, it would not be a break-action. Is your existing action already set up for external hammers? If so, then all you would have to do is modify the hammers to hit the offset nipples. If you use in-line bbls, you could use the existing breech without modification, but remember you can't hunt in this state with 209 primers, nor with enclosed caps. If I remember correctly, TC uses half-round/half-octagon bbls on some of their pieces.
There is enough meat in a black powder bbl to allow for dovetailing the lumps into the bbl(s.) Are you going to use a full stock, or a two piece? A full stock would make mounting the bbls easier, as the stock would hold the bbls in place and the action's recoil plate could be cut to receive the hooked breech plugs of the bbls.
There, that should be worth about $0.02.