Here's a good article that explains block casting numbers. The date code should be near the oil filter mount.
Deciphering Ford Parts Date Codes - FORDification.com
Based on your 5K26, the date is 19_5, October 26. The C on the head signifies the decade, 60's, so your engine was cast on October 26, 1965.
Since the model lines came on in the prior September, your engine was made for a '66 vehicle.