This is what I've been told, once upon a time--------------
Sideplates are (at least were, back in the day when these things were made by craftsmen) fitted by swaging. Specifically, the sideplate was set into the opening in the frame, and then whammity-bammed into place with a lead/babbit bar----more often than not with one mighty blow by the best of the best. Is that the way of it? Like I said, that's what I've been told---I wasn't there.
It strikes me the first order of business for this procedure is a fixture to hold/support the frame----lest it get bent. It strikes me also the next order of business is to smack the sideplate in EXACTLY the right place----lest it get bent too.
My knee-jerk reaction to all this is I think my first order of business if I needed to replace a sideplate would be to go buy a new gun.
Ralph Tremaine