Other than Super Duty the ‘63 & ‘64 engines both 389 and 421 equipped with the 9770716 heads were the first to have the rockers oiled through the pushrods.
The head gasket seals the unused oiling passage just fine, no need to plug it off.
Pictured below, 1964 389 & 421 blocks showing the rocker stud oiling passage.
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1964 Tempest Coupe LS3/4L70E/3.42
1964 Le Mans Convertible 421 HO/TH350/2.56
2002 WS6 Convertible LS1/4L60E/3.23
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