Car ID

How would one go about finding how many cars were produced in a particular model/configuration for any given year? I have the VIN, fender tag, and (somewhat legible and partially complete) build sheet. Since it's a factory '72 340 4 speed in a fairly unusual color (Winchester Gray/Gunmetal), I'm thinking it might be more rare than I originally assumed. Unfortunately, the original engine is long gone so it's not a matching numbers car, but the motor going in is a '72 period correct motor.