Looks like a '64 block with a later head on it. If someone put in plugs for a '64 head, that would explain your difficulty in getting a socket on. The '60-74 head used a plug with a 13/16 wrench size whereas the '75 and newer head uses the 5/8 wrench size. There's not enough room on the late head for a 13/16 plug and a socket unless it's a real thin wall and the plug isn't in all the way.