Slick 60's

I need help figuring out what year the motor in my '66 flareside is.
The PO said it was an '80 but I dont think so. Here are the numbers off of the block.

D5TE-60-8(or S)-AA

Can anyone help?

Tanks!!!!

Go here and look at there information guide on 300 6.

http://www.fordsix.com/castings.php

D5 is 1975 so its not any older than that.