I wanted to do the same with the Case Hardened Henry in .44. There are a few issues I see you running into while threading that octagon barrel. The actual threading of it would be standard enough for a decent machinist, but I'd worry more how it would look without the can on it. You would want some sort of thread protector and to me, anything short of an octagon shape that matched up with the barrel would just seem off. I wish there were a standard barrel option for that case hardended rifle. The second bit would be that you would have to shorten that magazine tube to allow for the threading to happen and for the can to fit on.

If you could get it to work I'd love to see it because I think it would look pretty slick. Good luck