The Stovebolt Site Forums The Shop Area Interiors Headlinder for 52 Chevy Truck

Here is the techtip for the chevy 2 piece cardboard headliner.
http://www.stovebolt.com/techtips/headliner_install_cardboard.html

On the AD chevys you need the metal on the back. The door windlass holds it in place on the sides. In the front the windshield garnish trim hold it in place.

...1950 headliner pic...

not sure if GMC is the same.
good luck