Back in the day, I'm sure the spots weren't known by many people. Now everyone seems to know. That's why documenting such changes would be extremely important if it if there's a question. Could be as simple as a build log showing the new cowl or rad support etc. Of course people will weld the originals in place or stamp new numbers in some cases.Back when I was still active in law enforcement I took a class on stolen vehicles. The instructor (a Federal Agent) told us that all U.S. made cars had the V.I.N. stamped somewhere on the frame or unibody that was not disclosed to the general public. He claimed that he could tell if a car had been 'rebadged' and if he found one that had, it was automatically seized and usually destroyed. I certainly wouldn't risk putting my money into that kind of endeavor!
There was an episode on GYC where MW looked over a 70 440-6 car. Turned out to be misrepresented as the engine was not original. That in an of itself could get someone in hot water if it's represented as being the factory engine. This particular engine had been re-stamped rather poorly but the average person would have little clue.