Not being an in-line person I can't give any advice on your breech plug. These inliner guys on here have been playing with theirs ever since BH209 came out because it is a bit harder to ignite. But right now, I think Grouse has the best idea to get you thru the hunting season. But I would go with Pyrodex RS over Triple 7. (I just never had any luck with T7)
You can worry about a breech modification later after the season.