P code Big Block Duster ????

A "P" code Duster is not a big block car. No such thing as a factory big block Duster.
The "P" code on the engine should indeed be a 400 from the mid to late 70's. I am doing this exact engine in a early Duster very soon. Those P code engines came in Magnum's, Cordobas, Fury, etc.... B bodies.