Paul is actually the one that had the inner roof patch panel reproduced. It is correct for the 67-71, but will work in a 72. The patch panel has the raised area where 67-71 the interior rear view mirror mounts. 72 mirror is mounted on the windshield.
http://gmcpauls.com/Patch%20Panels.htm
Chevy was stock coil rear, they could be ordered with rear leaf. GMC just the opposite and could be ordered with a coil rear.