assuming the original owner paperwork is gone forever ( factory records for Buick have mostly been lost ), the only way to 'verify' it would be to find the build sheet ... which might require a full disassembly of the car to even find.
sometimes they're over the fuel tank, sometimes they're in the car under the carpet. there have been other locations.
other than that, all you can do is try to match the various major components.
engine and trans will have portions of the VIN stamped on them.
engine, trans, rear, carb will have particular part #s and stampings. OEM big block cars have their own steering coupler.
a lot of that can be falsified if you have a particularly dedicated forger ... but that's part of the benefit of being in the Buick community. because Buicks tend not to command top dollar in the classic car market we are MUCH less of a target for that kind of thing.
the Stage 1 registry doesn't go back to 67 but you may find something useful on there.
http://www.stage1registry.com/GSinfo.html
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