According to the 1966 Pontiac Chassis Parts Catalog, Pontiac only supplied oversize pistons for the '63 326 (actually 336) to +0.030" = 3.810". Not saying you couldn't find larger pistons elsewhere.
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My Pontiac is a '57 GMC with its original 347" Pontiac V8 and dual-range Hydra-Matic.
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