WHEW!!! After long hours burning midnight oil, digging through thousands of pages, books and binders stacked six feet high, just for you, HERE is the answer:
C7ZZ-17906-A. Rear Bumper, for '67 and '68 Mustang Only.
--the '65-66 rear bumper is C5ZZ-17906-A --not for the '68 model Mustang.
-- the '69-'70 rear bumper is C9ZZ-17906-A -- not for the '68.
So...don't let any parts dealers (swap meet, EBAY, etc.) try to tell you that anything other than a '67-'68 rear bumper will work.
Paul N.