A hypothetical

I have an opportunity to possibly acquire another side by side 12 gauge.  This one is old old old, 1890's old with damascus barrels and loose headspace.  Which would require me to replace the hingepin, build up the hook or both to get it back into proper headspace without trying to sleeve the chambers which I DO NOT WANT TO DO!  As I've only done that once on a single shot 16 gauge and it was over 20 years ago when I did it..

IF, IF, IF I do manage to get this shotguna and I want it to be anything but a wall hanger and if I DO manage to correct the headspace, make a new firing pin for the right hand chamber and a new firing pin spring for said firing pin.  I will need to use ONLY low pressure black powder or black powder replacement loads in it.

I'm thinking since I don't want to mess with a chamber cast if I can get away with it, I'll have to use the 2.5" brass shotgun shells with an overpowder wad, felt wad, shot then an overshot card.  But I'm not sure of a few things.

I'll probably use Pyrodex rather than black powder as BP is almost impossible to find and I can still get Pyrodex RS for a decent price.  The brass shells use Large Pistol Primers, so I MIGHT have some ignition problems, unless I use Magnum primers.  Then there is the overshot card.  I can't use a roll crimp, so what type of crimp would I use?  or would I use the old standby of hot wax?  Also since Pyrodex is a volume for volume replacement NOT a weight for weight replacement for BP, just how much should I use?

Does anyone have load data for late 1800's 12 gauge shells?