What year? If it's 69-70, a 7/16-20 will work. Get one long enough to use a nylock on it. If you have a Tractor supply, they have graded bolts, unfortunately not in UNF though. If it's 71 or later, someone else will have to chime in. Look at Old Britts site, they have everything and usually tell you what it is.
Dave
69S