Nice car Steve... I have a '66 LeSabre Convertible. Unfortunately I can't help you much with your trunk problem. I was under the impression the the key hole was under the Buick emblem at the back. Mine is in plain sight.
From what I can see the lock cylinder is missing. Perhaps the latch mechanism is still in place. Look inside the hole using a flashlight and look for a vertical slot. Using a flat blade screw driver turn the slot clockwise. The trunk should pop open or might require a lift once you hear the click.