Determining the correct pulley is dependent upon many things. The dual sheave pulley pretty much disappeared after 1970. Most all engines starting in 1970 used one of the two different single sheave pulleys. Typically a dual sheave pulley was installed with a 55 amp alternator and without power steering.
*Restoration tip – Prior to final assembly of your alternator, verify that you have the correct alternator fan. 13 blade fans (10A310) were used until November 1969. There after, a 10 blade fan was installed.