So, if a car were to leave the factory and be sold at a dealership without its "original engine", would this have to be disclosed to potential buyers in order to avoid federal fraud charges?

To the factory, what is/was "original" and "matching numbers"?

Again, don't forget these cars were rushed to market to meet SCCA requirements and this was the 1970 model year which broke sales records for Chrysler across the board. Having woked in factory environments, I have no doubt that shortcuts were taken and rules broken. I can almost hear bosses from the top down screaming "Who cares? Just get the damn cars out the door!"