Let me put it this way:
The 383 with 2X4V in-line carburetion on the '59 Dodge D-500 was listed as having 345 HP and the DeSoto Adventurer engine that year (with a different paint scheme) was given 350 horses, but both engines came out of just one plant. In 1960, the standard ram 413 on the 300F (not the 300F “Special”) was advertised at 375 HP. It stands to reason that since the 383 has 93% of the displacement of the 413, with virtually the same internal components (i.e., valve gear, heads, cam, piston configuration, exhaust, etc. without taking into account less friction), it should develop 93% of the horsepower of the bigger mill, or some 350-360 ponies. This corresponds to the 345/350 of the '59 high performance 383s. I think it rather odd that there be a power reduction in the early days of the horsepower wars. I have it from some rather reputable sources that there was some rather nasty internal corporate politicking going on at the time regarding lowly Dart and Plymouth having that much power available, particularly with the image the Plymouth Division wished to present (read “STAID”). In fact, the intended big engine for Plymouth in 1960 was supposed to be the ram 361 and the ram 383 was made available later in the model year only because of pressure from Plymouth dealers – and even at that, the “Plymmer’s” ram 361 supposedly had 310 ponies, while the identical engine in the Dart somehow got 10 more, or 320 (different colors but same engine from the same plant), Interestingly, the 2X4V in-line carbed 361 in 1958 had 325 horses.
Incidently, the internal passages on those old “Long Ram” were often modified by cutting away the internal passages to maximize the top end performance. Plymouth produced only 1577 ram cars (361s and 383s – in about equal numbers), but I don’t know the Dart figures nor that for the '61 model year. I can speak from some experience on these cars as I not only have a “SonoRamic Commando” '60 Fury in my stable now, but I also campaigned a similar car back in 1960-1962.