Federal Law mandates that if you perminately change/modify any Replica/original C&B ... ect. it is illegal to sell said Revolver. But does not say under Federal Law that you must register it ... that would include grinding out the right side of the frame to rear load a Konvertor such as a Kirst gated or not... and thats my limited education on it.
That's not what the law says. The law says that you cannot engage in the business of manufacturing firearms unless you are licensed. The law also defines what being engaged in the business is. If you're just a guy who owns a C&B gun and installs a cartridge converter in it, you're not engaged in the business, even if you later sell it.
Read 18USC, Chapter 44, paragraphs 921 and 922 for the details.
And, on the topic, sort of, I think that Magnum Research makes a .45-70 revolver. I've never seen one in action, but the pictures scared me plenty!