Andy,
I have a '64 Caddy with pretty much the same frame by the look of it. It must have been the way to go across the GM range in the 60's, except for the Corvette. I have two '74 Corvettes, a coupe and a convertible with both tops. Plus of course, a '67 GMC stepside. Nice car you have, by the way.
Regards from Down Under.
aussiejohn