I am thinking about a similar idea on a scabby old Jones Underlever Paton shotgun, but its really hard to think of a big and yet low-powered old cartridge that would suit. I think you might be better choosing a more modest BP calibre - maybe a 44/40 or if that's too anaemic for you, a .450 No 1 Musket.Maybe .577 Snider would be a good choice, or a 20 bore rifle.

The thing is a 12G barrel at the breech has a whopping diameter.