It's hard to really know without an in-person inspection, but from what has been shown, it's is a Desert Gold '70 Skylark 350 2 door hardtop (43537) that had a vinyl top and a Burnished Saddle bench seat. It's not a GS 350 or even a GS 455. The unpunched firewall seems a bit strange, but unless I saw it in person, or better pictures with ALL the paint striped off, I would be willing to bet a coke someone just did a nice job of welding and filling the hole so their racecar would look nice under the hood. We've all seen show cars with smooth firewalls, and even a couple of faked cars that had the "secret" VIN welded very neatly into another car, it's not that tough. Maybe someone had a connection to Kenne-Bell and added the plate. No one on the Buick board seems to have ever seen "COPO" associated with a special Buick.
With the VIN and $50, the mystery could be at least partially solved by contacting the Sloan Museum. They have the records for all Fremont built 70 Buicks and can supply the exact list of options, dealer number (and usually the dealer name) as well as the invoice price etc for this specific car. If there truly was a special Buick built in 1970, it would have come from the Flint plant. Heck you couldn't even get a Stage 1 car built in Fremont in '70 (all '70 Stage 1s were built in Flint).
http://www.sloanmuseum.com/research_form.html