rear quarter panel , the padded upholstery area just below the rear quarter window on a ' 69 Cutlass convert, ... The new vynyl peice has a cardboard peice glued to part of it , while the old original peice has the card board also but it is also glued and rivited to the top curved metal part . Obviously I need to keep the top curved metal peice, so , do I remove the original metal peice from it's original card board, ??? and if so , how do I refasten it to the new card board ? OR, do I leave the original card board and curved metal peice together and just add (glue) the new card board over the old one ?

 

The correct way would be to remove the old curved metal part (called the rail) and install it onto your new panel.
I did this to the front door panels on my car, the concept is the same. Legendary reworked my rear panels for me.

You might get some rework hints from these posts:
https://classicoldsmobile.com/forums...tml#post162232

https://classicoldsmobile.com/forums...html#post66668

 

thanks, I did it the other way around, had them do the door panels and I'm doing the rears ........

 

would a glue such as the Gorilla Glue work for gluing the rail to the panel ? How about using head liner spray type glue for the rest of the panel ??

 

Headliner spray? If you mean 3M 8090 adhesive, yes. Do not use anything else, as that is the only one I know of that will take the heat of automotive use and not let go.
I learned the hard way...