In 1974, when GM was forced to produce engines that would run on unleaded fuel, many changes were made to the cylinder heads. The addition of an extra heat crossover port on the center exhaust port and the redesigned exhaust port did reduce emissions, and raised the operating temperature. GM also began flame-hardening the exhaust seats to keep them from eroding under the new unleaded fuel conditions. On top of that, GM went on a lightening binge. All these changes contributed to a rash of cracking cylinder heads. Cracks usually formed around the exhaust seats of the center cylinder. The new exhaust port configuration did not improve flow. The most common casting you will find is the 882 for the 350ci engine and the 454 for the 305ci engine. These castings have a strangely cast area under the intake ports which make them very easy to spot. These castings have a 76cc chamber.