Ideally, both numbers would be best to get an accurate ID. The casting number JiMerit mentions will get you in the ballpark, but the stamped number behind the distributor that waldo53 mentions will give the original application. However, if the engine was a service replacement that was installed after the truck was originally sold, then it may not have that number, as the usually were not stamped.
Once you get the casting numbers, check
here to decode them. If you have the serial number from behind the distributor, post it here. I have the truck numbers (and some pasenger car) up to '72.
