421 SUPER DUTY ON THE DYNO @ Butler Performance!! This 421 SD is using the early "306" heads. These flow a little less than the 1962/ #127's and much less than the 1963/ #980 heads, but it still made great power. Almost 500 HP and well over 500 ft.lbs. of torque!! We actually have both a 1962 389 SD with #127 heads and 1963 421 SD with #980 heads in the shop now that we'll be finishing soon. We love working on these old, rare SD's and really love to hear them scream on the dyno! #ButlerPerforman