I believe you should have a 1/2-16X6 1/2 hex head grade 5 bolt. A quick look at the heads of the one I have show a "F" in the center of a grade 5 indicator. This is the bolt, 380328-S8, also used in 1967 and I've found them on several of my 66 Mustangs, although documentation for this bolt for 66 applications is not available. There must have been a running change, at least in San Jose. A different bolt was used in 1968 on, and a different bolt used for 1964-65. Ford don't make it easy.
Jim