Jim, if it's the engine that was in there when I sold you the car, it is factory.
When I dropped the transmission pan about twenty years ago, I saw a 1983 casting date. Original owner had told me it was replaced.
Seems like there would be a partial VIN on the engine block, but maybe PMD discontinued that practice.
I wonder if you had a PHS and cross referenced the dates & option codes with the body tag, which shows build date, color code, and
rare full vinyl top, would the police accept that as enough 'proof'?
Probably not likely, but just a thought.