Also - On my neighbor's 64 Pontiac, there is a VIN stamped on TOP of the frame under the trunk. Checking the net, seems Buick may have done the same thing. One quote found online:
Between the body and frame just forward of where the parking brake cable goes thru the frame and between body and frame about six inches rear of the second to last body mount.
In my neighbor's case, his GTO had thoughtfully rusted the trunk floor enough that he could peel it back...and there it was.