Short throated H&R 45-70s are common, have been for a long time, there are hundreds of discussions on it. With good neck tension, there's probably no problem with leaving a bullet stuck in the throat as long as the round extracts ok, would be more of a problem on an ejector barrel, spending $10 for a Lee factory crimp die would eliminate the possibility tho. As long as you're working the load up with the same COL with the bullet into the lands from the get go, should be fine, loading cast bullets to engrave slightly on chambering is commonly used to improve accuracy.
Tim