1967 Formuls S 383 ?? Factory upgrade?

Too many differences to be a dealer "upgrade"- not just an engine, but a transmission, radiator, torsion bars, K frame, exhaust, and a host of other stuff. The factory did install 383s in '67, but the VIN, fender tag and build sheets reflect the way the factory built that particular car- and you say it decodes as a 273.
I'd steer clear. The seller is either badly misinformed or a scammer.