Yes, the new tang/breech plug will need to be fitted or timed to line up with your top flat and also have the breech plug face fit snugly against the breech chamber shoulder of the barrel.
It's not a terribly hard job but it will try your patience. The first brrech plug I fitted took me several hours to complete. If you have a long straight tang it will be much easier, in my opinion, to modify the tang rather than fit a new breech plug.
Ken