My 76 450-SL has the same type of parcel shelf (I think), and I fashioned a small hook to insert at the rear edge and lift, there are 2 metal straps that hold the front edge with tension and with the rear edge lifted simply push the front edge to the rear and lift it out. When I removed my shelf for the first time I found seat belts installed under it. Who knows what you'll find. Good luck
Bob