A-Body Trunk Filler Panel How To Remove It

The trunk filler panel is spot welded to the package tray pan along the trunk gutter lip (under the weather-strip) as well as in the window channel and the seam on each side where it mates with the quarter panel. Here an inexpensive Harbor Freight spot weld cutter was used (note- these are fairly priced and last much longer than you would expect) along the trunk lip and diagonal across to the top of the quarter panel. A spot weld cutter could not be used in the window channel because pitting was so bad that the spot welds couldn’t be located to drill. 
The trunk lip as well as rear window channel spot welds are about 3” apart and can be located by wire wheeling the channels to expose the deformations in the metal at each weld. The filler panel is also welded in the trunk valley diagonally up to the quarter panel as well as in the “drop” between the quarter and the trunk filler panel.
The welds at the seam between the quarter and the trunk filler are shown by the green arrows and are hidden as they are on the drop tabs created by the drops of both panels. Because the filler panel is being sacrificed, a cut with a wheel was made at the dashed blue line and then those drop spot welds can then be accessed to grind or drill afterwards. The red arrows are spot welds that are to be drilled and the yellow arrow shows where an external weld was placed - it is odd, and perhaps not on all A-Body cars, but watch for it. These small welds can be cut with a wheel as well.
If your window channel was pitted as this one was then you really just have to start grinding away the channel bottom where the welds were placed to find the spot welds. Be very careful doing this as you can easily go through the trunk filler panel metal into the package tray metal and you don’t want to do that. With all the welds broken free you can then peel off the filler panel to expose your package tray.