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Measuring downward from the secondary bore of the carburetor flange to the floor of the deep plenum, we compared a typical '68 cast-iron casting (No. 9790140), an original-dated '71 455 H.O. (No. 483674) casting, and a '72 455 H.O. casting (No. 488945). The plenum floor of the '72 455 H.O. (and all S/R units) is roughly 0.1 inch higher, subsequently reducing airflow by 1 percent.