Depends on what engine you have, what component you are talking about and where your car was built. Base engines should be fiarly close to the cars assembly date. Within a day to a week or two for Norwood cars. LA cars could be a little more since things had to be shipped to that plant. Optional engines are another story and their components might have a little more of a spread than the base engines. High perfromance engines can be a month or two before the cars assembly date. But again, it all depends on what component you're referring to. Some things like exhaust manifolds were shared between the low end engines and the high perfromance ones. Holley carbs were just for the high performance engines and were built in batches, so they might be up to a few months previous of the cars build. See what I mean?
John Hinckley (JohnZ) can answer this better than anyone so I'll let him reply.