Not quite done yet....
Just rec'd the JBL replacements and minus a cut here and a resolder there, the problem I now have is that these are flush mount speakers which are meant to be held in place by way of a big (2.5" ?) plastic nut from behind. Hopefully you see from the pics.
The originals are of course held by two screws. Is there some kind adapter I can simply attach to the replacements?