Which box are you using to replace the factory spark box? I'm using the stock Duraspark system in my 77 Comet, but so far, I'm not at all satisfied with it, compared to the Pertronix II and Crane XR-1 I've used in my 89 Ranger. I did pick up some better results with a wire swap ( to Ford Racing 9mm's) and a Blaster 3 coil. Haven't run it enough to tell if that's going to clear up the missfire it was getting though. It did sound somewhat better though. Hopefully I'll get the oil pan leaks fixed enough this weekend to allow some street running to burn off some of the carbon on the plugs.