Pretty ambitious to get the 45/70 rim through the box magazine. I do not have a receiver in hand to see how much would be needed to remove. Going by the SAAMI drawing the case head diameter is .618 and can be .012 larger, that is now larger than the bolt diameter (.625). So the rim would have to come off the bolt. Not sure if the extractor could be made to work either. The rim is shaped to let aid the extractor to ride over the rim on the 30/40, the bevel helps to prevent jams in case the they are accidentally loaded out of echelon. We had a discussion on this recently. Do get your ducks in a row before making any modifications that can not be undone. Please print the drawings as I will remove them shorty.
Also watch for clearance between the extractor and internal threads in the receiver. As the extractor rides over the rim it may be close to the threads.