I can't answer on the '70 RR 383 w/AC - my only '70 B was a Satellite wagon, 383 4-bbl, AC car - so engine was not orange.
If it's the original engine & has never been rebuilt or repainted, one could of course assume whatever color's on it is factory.
Whether that was an error or is correct is another kettle of fish.
A lot can happen in 50+ years - back in the day, lots of mods got done...I painted my 318 Swinger's grille like a GTS, for example.
And a guy I knew with a base '68 RR painted the lower bower body like a '69 GTX & added the '69 V21 hood stripes.