One thing I've noticed is that the official vehicles' idle sounds are too quiet. For instance, listen to this 1973 Bruckell Moonhawk Oldsmobile Cutlass Supreme: The Moonhawk in BeamNG barely has any noticeable idle, which is unfortunate.
The car in the video has an aftermarket exhaust system and what sounds like a performance camshaft. Mid-late 70s land yachts did not sound like that from the factory. If anything, the Moonhawk sounds too aggressive for a stock 70s car. Stock Moonhawk would probably sound more like this
It's not so much the aggression, it's the volume. The Moonhawk doesn't even sound like that - it's more like a dull hum. It almost sounds like a duller, smoother version of my dad's Subaru.