i assume you must be minus the door tag? there are date stampings in the body panels,they will not be exact .but they should get you close. if i remember correctly they give the day, month and shift the panels were made. assembly was usualy within that month. an exact day would be hard without the door tag. i have actualy found a printed date on the windshield whiper motor and some times the bracket.
i have found a build sheet wraped around the wireing inside the dash ,you may get lucky