413 Coronet, the 67 383 manifold is indeed a low rise compared to the 68-69. I have owned 4-67 383 Darts and I have no idea where you got your information. They were indeed rated at 280 HP and the drivers side exhaust manifold was restricted due to it having to wrap around the steering column. In 1968 the manifold was opened up a little more and again in 1969, hence the different part numbers for each year. Also the AFB in 67 had less CFM than the AVS in 68 and 69.