Hi, I am trying to find out when ford changed stud sizes on the 300 I-6. My 77 came with self locking nuts. I need a valve job and I picked up another head which has smaller non-locking nuts and jam nuts. I can't make out the code on the head and am trying to figure out when they changed so I know the year of the head. They both have adjustable rockers and hopefully will interchange. Thanks in advance.