OK, my apprehension about polishing/filing/'fitting' the
rear of the sideplate stems from the process of moving the centerline of the trigger lever closer to the frame/sear. Because of the 90º shape of this lever, this can affect the sear engagement.
I don't have an armorer's oversize sideplate, although one of those [maybe two] shown above are aftermarket pieces and may be oversize. But the sideplate with the too-thick front tang seems most promising.
While I understand that trigger pull is important to some shooters, right now I just want to get the plate fitted. I'll adjust for pull later.
The length of the front tang
[where it hits the takedown lever] hadn't occurred to me (I was more concerned about thickness). I'll check that today.
Ed Tinker also commented that the takedown lever affects the sideplate fit. I don't have but one of those. Something for me to look for.
Always good to get a number of opinions and justifications on a subject I know nothing about. I thank you all!