Chrysler engineers figured that with the 50 or so added HP over a 383 Dart, and a guy hammering gears with a 4 speed rocket ship like these cars were, it mite result in a lot of un-necessary broken parts. The Torqueflite would be easier on things and a wiser chose. Remember these cars did have at least a 12 month, 12,000 mile warranty, that Chrysler would have pay to fix things. Plus with that huge 440 RB motor in the Darts small engine bay, plant engineers figured there was little to no room for clutch linkages and such.