The gear type was used on 401 through 478 engines, the double row bicycle chain and sprockets were used on 305 through 379 engines. There is a difference in years with the chain driven setup as the tooth count was different from 1960-65 vs 1966-74. The 305E was introduced in 1963, the difference from the 305D was a new ventilation system and the previous 305A was a 1bbl carburetor.
