The earliest 265 and 283 small-blocks relied on an iron Power Pack head that featured 58-cc chambers and tiny 1.72/1.50-inch valves. The bigger heads to follow were the “double-hump” heads championed by the mid-’60s 327 engines. The easiest way to reference any production head is with the last three digits of its casting number.

These types of heads will have 3 factory-drilled holes on the driver side head, and 3 holes on the passenger side head with the same dimension as the top two holes on both sides of the 'Aftermarket Aluminum or Cast Iron' heads.