It's a crossmember mounted between the frame rails just below the trunk floor above and slightly behind the rear diff. It works perfectly and it doesn't intrude into the trunk floor.
This same style shock mount has been sold and used for first gens to eliminate the stagered shock set up with bigger tires. All the aftermarket suspension companies offer a similiar setup it's nothing new, Jegs sells them
Competition Engineering 3019
67-69 Camaro/Firebird Rear Shock Crossmember