or 'Trunk floor drop-off', or just 'drop-offs'.
There are repos for GTO avail, dunno if they fit. Chevelles have a shorter trunk length than Buick, Olds Pontiac..... at least for 66-67.
Or make your own panels. I have one of the Harbor Freight bead rollers. It works pretty good for rolling beads into patch panels. At about $100, it'll pay for itself after doing one car.