I thought Caddy handled noise with extra sound insulation between the floorboards and the carpet. It seems to work pretty well - I had the chance to ride in a pretty mint '76 Fleetwood a while back at 80mph, and that thing didn't seem to know what road noise was (I don't remember what tires he had on the car, but they didn't make any noise either). I'd hate to drive that thing on a long night trip, because I'd probably fall asleep in that cocoon, and wreck that beautiful ride. It's beautiful black, it's the last year of that generation, and he has my number if he ever wants to sell it.
That being said, some of the high-dolar resto purists (the kind that can easily afford to pay somebody else to rotisserie their car) would rather run 2-wheel cable-operated brakes on their classic car, if that was original equipment, rather than modern brakes. Better smashed into a brick wall than not, if they stayed original seems to be the mindset of some.
What does this have to do with sound-coating on the underside and never seeing it on classic cars? Part of that could be that if they do use it, they might put in on the inside instead, or part of it could be "better noisy than un-original", or they might want to see any rust easily at a very early surface stage when it could be easily fixed. Some wouldn't use it just because it wasn't OE.
