as you may already know, A-bodies never had 3 point belts for rear seat, less it was late 75-76 4dr cars (maybe). So, any slot you are seeing in the rear package tray would not be designed for a seat belt. My guess is you are seeing the optional rear air/rear defrost vent/duct slot thats in every a-body.
Id suggest looking to hard mount the 3rd shoulder belt in the sail panel area (similar to most moden cars), but it has to mount to heavy structural metal, backed up by backer plate or similar, otherwise in an accident, it is simply going to pull from the mount/rip out. This leaves you with an issue for middle shoulder belt however, if you want all 3 seat positions to have 3 pnt.
My only suggestion there is to go back down to the floor, but there is no legit way to spread the dynamic loading. Package tray wont do it, in fact if not done right, that thin rear shelf metal may just cut a belt or crumple, under the right conditions. You may need to do something like a custom harness bar. They normally work/are sold for front seats running 4 & 5 point race belts, but its a thought. Mount in in the trunk just behind the rear seat back and your set.
