If a GMC block, the engine serial number will start with the engine size, 288, 316, 347 or 336 depending on the year. And no, I can't tell you why GMC advertised one cubic inch larger than Pontiac in 1955 and one smaller in '56
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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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