I'm looking at pictures of the rack in question. In the 1966 accessory brochure they show it on a Chevelle SS396 convertible, and in my 1975 accessory brochure it is shown on the trunk of a '74 Camaro (go figure!). It does not give any dimensions, but they really do appear to be nearly identical! The only difference I can see is that the '66 has the rear legs as part of a loop that forms the rear edge of the rack; whereas the '74 piece has the rear legs mounted on the rear part of the side bar (actually part of the U-shaped frame), and the rear edge seems to be a separate bar brazed onto the side bars. In both cases the pictures are artist's renderings and as such may not be accurate. They really do look nearly identical, though. Both use 6 tubes that run back from the main U-shaped frame, and both appear to be suspended at the rear by vertical bars that attach to the rear crossmember. Ah, trivia!
It sounds like you might have a line on a somewhat rare accessory; go for it!