To further go into the numbers thing, most Ford parts had casting numbers on them. They mixed and matched a lot of stuff from earlier models when possible, but always used the same items in a given year and model, unless there was a midyear change, and then everything after that used the new number. As an example they may have used a manifold from a 66 Fairlane (coded C6OE) and while a Galaxie manifold from the same year might fit (coded C6AE) it would be technically incorrect. For a 68 Mustang, anything with a C9 or above is obviously wrong, but it will probably work just fine. Without the big Ford book to tell you what is correct, you'll go nuts trying to get it all right.
Got you totally confused yet?
Steve