How can you tell if the motor is number matching on a 1965 Plymouth Belvedere? Also how much does it hurt the value of 1965 Belvedere if it has a 440 in it instead of the 383 that it was born with?

My opinion is that if the car is otherwise perfectly restored BUT has a different size engine than it was built with, the difference in value could be up to 25%. If several changes were made with the car such as a color change, added or removed A/C, bigger wheels and tires and so on, I doubt that a different motor matters. The horse is already out of the barn by then.