Minor clarifications..
The first Plymouth only engine, the 'A' series, was 1956 at 277 cubic inch.
The Dodge based Poly engines, 241 and 270 overlaped the A engine into 1956 in the Plymouth car line.
The Chrysler 'Poly' engine was the Spitfire and was first used in the 1955 Windsor as a 301 ci. This is a poly head on a Hemi block. This is NOT the same as the Plymouth 301 which is an A engine. In 1956 the Spitfie used the 331 block and in '57 and '58 was a 354.
The Dodge based Poly can be converted to Hemi.
The Spitfire can be converted to Hemi.
The Plymouth A series cannot be converted.