Beginning in 1969,the Vin number did not callout the original engine size it was assembled with.The tachometer was an extra cost option on either Super Sports OR Malibus,so that's no indicator.
Among other subtle details,ask him what the rearend code suffix and assembly date of it is,since he's claiming it as authentic to this Chevelle.
If he has no original documents to show what it was built as,I would look into past owner history and see what they say.
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They Will Reign in Arizona..