71 Dodge Dart Swinger in England

Yep the shaft seems ok, I can get an even 8 thou reluctor clearance in all positions. Im now rethinking the headers, which I have had off but still have a slight leak at the head gasket. Could this cause this symptom? They are hedman headers and fit badly seems to hit everything in the engine bay and are poorly made. I'm tempted to go back to cast stock manifolds. if I can find some in UK.