Make sure that no two CONSECUTIVE firing plug wires are touching each other.
Unless they run thru a METAL tube then it doesn't matter.
Keith (PackardV8), is this evidenced or an assumption? Often ignition cables touch each other, regardless whether metal or plastic-covered. The attached pic shows the even rail-guided ZIS-system, cribbed from Packard. I think, the problem starts when the ignition cables begin to crumble. Once I have learned that the cables can be measured. The upper limit revolves around 25kΩ.