Just to clarify something in your original post.
"This may validate the common theory that the factories that built these trucks pretty much did what the wanted as far as engine colors..."
You do understand ? that there were only two engine plants building and painting engines at this time , Flint and Tonawanda . Yours is most likely, Flint, where the engine was built start to finish including paint. Engines were not painted on or at the many assembly lines where these trucks were built.
Heres some info on engine assembly.
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