Coldsteel - I think you will find that gun (beautiful, btw) to be regulated for conicals. This conclusion is due to the sight arangement. Had it only the leaf sights, then I'd say, probably round ball. The addition of the ladder sight pretty much shows this double to be regulated for conicals. Now, I'd go ahead with testing using round ball though, due to the twist rates.
; I'd probably radius the crowns in order to allow loading and shooting a more snug ball/patch combo, than what the sharp crowns will allow. Sharp crowns are prone to cutting the patch on loading. Perhaps a wad first on top of the powder, then the patched ball might shoot OK. Your requirement of 4" at 100 yards may not be possible with those sharp crowns, but might be, with a good radius. Fine emery cloth and your thumb (or large bearing, then your thumbb) will do the job perfectly & look 'stock'.