Since writing the book back in 2016, we've discovered new information on a few different points. One of those is the #12/#13 cylinder head usage in 1970 Firebird. A 1970 engine assembly document that I located clarifies without question.
The #12 heads were indeed used on the WT 400 (along WS and YZ codes) engines that year. The difference between the #12 and #13 (besides chamber volume) was the inner valve spring. Both heads used 9779009 outer springs. The #13 used 9779008 inner while #12 used 9781476 inner, which contained a higher rate.
Pontiac was clearly looking to increase high rpm valvetrain stability with the manual transmission in the WT application.
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