I have a 1956 Mercury Montclair with a 312 and automatic transmission.  I know the car originally came with a 312, but I'm trying to figure out if the engine that is currently in the car is the "original" engine that came with the car or if has been replaced at some point throughout the years.  Can anyone point me in the right direction as to where to look and what numbers I need to compare?  Thanks a bunch.