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WRONG. All Pontiacs from 326-455 are the same size block externally. The only major difference is that the 326, 350, 389, and 400 have a 3″ main bearing size. The 421, 428, and 455 are 3.25″ mains. Now…if you are talking about comparing a Pontiac block to say a Chevy block….the Pontiac is larger than a small block and slightly smaller than a big block.
