The deck hieght, combustion chamber size and head gasket thickness will determinge the true mechanical compression ratio The OEM 1963 415 HP pistons had a .063 high dome, not sure of the dome volume, 1964 415 HP pistons had right at .120 dome height, same thing on the dome volume sizes I've never seen a OEM production Mopar V8 motor, regardless of which size or family, have anywhere close to factory deck heights, combustion chamber size or anythig close to the advertised compression ratio My message is, check yours very closely to get exactly what you want