Ford Car Parts (aka MPC), 75 ed, section 86.1, says you need Ford Locator Code number JC-607 (service number C9PZ-8620-DE) a 1/2 inch wide, 60 7/8 "long" belt. Check your lower pulley diameter too. The outer sheath (of three) should be a smaller diameter than the two inner ones.
(The 1966 Ford Products Engine Equipment - - (AM0156 pg76) shows double inner belts, and the outer (A/C and smog) as C6ZE-8620-C, 1/2 X 60.00 inch, but that number does not appear in the MPC.)
Jim