Question about early 60's assembly line changeups.

Hi folks. I've got a 64 Valiant with a 273 V8. By all accounts so far as vin decoders say it's supposed to have a slant six in it. Personally I would think if anyone was going to swap a six for an eight they would put something later in there but this is a 64-65 style 273.
So my question is, did they make decisions during assembly to put different engines/transmissions in a car than was ordered? If so could they have mistakenly put the six cylinder vin tag on it? I know all manufacturers did some crazy things on the line back then so I wouldn't doubt an engine change mid build. Any thoughts?
Thanks, Wally.