A suggestion for when you go to black powder, or maybe before. Over at the Marlinowners.com forum, you'll find a forum for "Ranch Dog Molds" he has some molds he custom designed with Lee Precission, that cast .460dia bullets in 350 and 425gr. I have both molds for my marlin Guide Gun, but haven't shot them yet. However they get rave reviews over there.
Since marlin makes the H&R, and since my BC slugged a bit large as well, they would most likely be the perfect fit for the BC bore.
Congrats on the new BC.