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No, 331 Chrysler and 318 poly are completely different engines with different bore spacing (i.e., different distance between cylinder bore centers).

You can put Chrysler hemi heads on a Chrysler poly, and Dodge hemi heads on a Dodge poly.

There are no poly versions of the De Soto hemi, and no hemi versions of the Plymouth poly,
BUT some 55-6 Plymouths and some 57 De Sotos used Dodge poly engines and you CAN put Dodge hemi heads on them.

The 56-8 Plymouth poly became the corporate Chyrsler small block in 59, used in both Plymouth and Dodge from then on; later developed into the 318 wedge and used eventually in not only Plymouths and Dodges, but also Chryslers, BUT, it was originally developed by Plymouth for Plymouths.

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Ok, now I think I'm going to throw up from my head spinning so much!