I thought that I had nailed this topic Greg but it is elusive as the numbers around the motor and its components are indicative of the design date not necessarily the actual build date of the motor. If you have access the UK Kitcars mag "Cobra" issue 15 has a good article on Ford motor dating. The block or casting number is usually up under the starter motor - mean under it and the starter may need to be removed to see it easily. There will also be a number on the heads.
There are many websites that try to guide here but this one is quite useful:
http://www.classicmustang.com/decoding_part_numbers.htm
I could scan and post the "Cobra" article but would not have time until next week.
Hope this helps,
Merv