Sorry but a 383 roadrunner would not handle a 340 dart with the same transmission and gear ratios the dart would win every time. My brother had a 70 roadrunner 383 4 speed and I had a 340 swinger and it broke his heart every time we raced. The 383 was rated at a higher horsepower because they did not want to have the 340 cars cutting into the b-body platform sales for that reason they kept the horsepower rating down on the 340 fact. that being said the 383 is still as mentioned a more rigid platform and will handle more horsepower
