I scanned through the posts in the thread and didn't notice anyone mentioning the fact that the 69 335hp 383 motor had positive deck height flat top pistons originally. It wasn't much but it kept anyone from just doing a quick bolt on of the 67 440 closed chamber heads to increase compression. You had to make sure your head gasket was thick enough to keep the piston from hitting the combustion chamber. When rebuilding the engines in the 70's, the catalog had a listing for both pistons, neutral and positive height. Consider this as a possibility as to why the horsepower rating was different. Eventually the taller pistons were just dropped from the books and like now only one is shown.

I recall this was one of the tips mentioned in the big yellow Direction Connection Racing Manual.

Perhaps Plymouth converted to the open chamber heads for emissions or some other Federal Mandate and the piston change was a little cheat to help performance.