Jason, The easiest way I know is to pull a valve cover to look at the casting # on the head. Also there will be a date casting # starting with a letter, then three numbers. Example: C 15 0= March 15, 1970. The 1970 head uses a gasketed 13/16 spark plug and 1971 head uses a 5/8 tapered plug. The gap should be set at .035 on either spark plug. Once you have the number, check it out on this site http://outintheshop.com/faq/casting/heads.html
. to find out what you have.