Bench Racer Challenge.
I have, in the past, posted the dyno results from a 440 with mildly ported 516 heads and the Mopar 509 equal of that camshaft.......449HP. If we bump the compression on the 440 from about 8:1 to 10:1 we would see, maybe, 485HP. Perhaps the 383 felt like 600HP but it wasn't making 600HP.
If the Dart only weighed 2500# and the 383 made 460 FWHP..........maybe the ET claim is accurate.