1967 Formuls S 383 ?? Factory upgrade?

So I found a 1967 convertible Formula S but the vin reads 273…..
Current owner claims that he had engine “upgraded” to the 383 by Plymouth…
Has anyone ever heard of the factory offering engine upgrades like this?
Were the big blocks installed on the line originally? I know the hemis were an outside company.
Any input would be appreciated. He’s asking $30k
He should have a whole bunch of receipts and documentation for that fairy tale.