They’re all definitely 3/8-16 UNC thread, Pontiac never deviated from this exhaust manifold fastener thread size.
It’s easy to find whatever bolts you need at a hardware store. Just make sure the bolts you use aren’t so long that they bottom out. Keep in mind that the end bolt holes intersect with the head bolts.
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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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