T answer your question instead of giving an opinion that was not solicited ...

Yes it could be done , wit hall the interior parts pulled away and the headliner loose you can see where the post would go since much of the sheetmetal is shared between the 2 body styles .

You would need a number of interior parts also , the filler under the rear window , the strip that goes along the top of the pinch weld for the head liner , the roof rail weather strip and the the piece of SS that holds it. There is difference in the under structure , exactly what ??? A post coupe is lighter than a hardtop , there are structural parts added to the hardtop that aren't needed on the post car .