MPC places the change over as the first of Feb 69. Not sure if that is a hard (real cut off date) as this was not something that was related to safety or an issue that needed correcting. Also if that were a hard date the date would likely correspond to when the engine was built not the car
IMHO use it as a range since there is no telling if a car coming down the line would have received an engine with the chrome or aluminum valve covers if at that exact moment were both sitting there waiting for cars. The overlap would not have been great but it does mean that for a period there was an overlap