First let me give you an opinion Robert - a correctly dated engine, since it is not original to the car anyway, adds no value to the car. You could drive yourself nuts trying to track one down I suppose. Just my two cents worth.
If you do decide to go that route, here is what you should be looking for.
Engine - Yes, you would want a December, 1968 engine or early January, 1969. A November 1968 would probably still be in the range, but over eight weeks might be an abnormal stretch.
M20 - These are dated totally different than the engine. Here is an example of one coded P9A04A:
P - Muncie.
9 - 1969 (8 - 1968).
A - January (T - December) (a lot of letters were skipped).
04 - 4th of month (31 - 31st of month).
A - M20 (B - M21, C - M22).
Hope this helps a bit.