I have checked several of my resources, and only found 3 that address your application exactly; they are the 1967 Master Parts Catalog (dated January of 1967); the 1960-68 MPC (dated June 1968); and the 1965-72 MPC (dated May 1975). Unbelievably, all 3 agree. This is unusual as by the 1975 printing, many parts have typically been superseded by newer parts that "will work" but not be indicative of the original part.
All 3 show Service Part # C4AZ-6A312-B as being for a 1967, 390 Mustang, with A/C, with or without Power Steering and without Thermactor Emission. Further, the first 2 sources noted, both show this as a Triple Sheave Pulley, the outer one being 7-3/4" O.D., and the inner one being 6-7/64" O.D. Further, they note that it uses one 1/2" belt and two 3/8" belts.
Unfortunately, none list a casting number for this pulley. I have not checked the Engine Assembly Manual, but possibly it may note some Engineering/Assembly Part Numbers.
Sorry I couldn't find more.
Bob