Single-leaf springs are fine as long as they aren't subjected to wheel hop. Many high-horsepower cars have run single-leafs with no problem--some drag racers actually prefer them over multi-leafs. The '67s didn't have staggered shocks which compounded the problem with the single-leaf springs. Without traction bars the single-leafs would literally bend into an S-shape then snap back, causing violent wheel hop. After enough abuse they would break. The problem wasn't with the single-leaf springs themselves, but with the lack of any traction control to stop the windup on hard acceleration from a standing start. Some '67s had a traction arm that was supposed to help, but the only real solution was slapper-type traction bars or ladder bars. Hope this helps and good luck!