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They made the 354 Hemi and the 354 poly at the same time but I think most of the 354 Polys went to Canada.
The 1958 354 engine was available in both Hemi and Poly form; with four barrel carburetors, the Hemi produced 350 horsepower, and the Poly just 310. Still, the Poly was much cheaper and lighter, and the company could make larger engines to replace the lost power