Hi 62,
When you turn the ignition to start, there should be full battery voltage on the "S" (start terminal). Keep in mind if your starter is engaged and there is power to it, your battery voltage will drop to around 9 volts depending on your engines demand (load) on the starter. The "I" (ignition terminal), like Twisted6 said, sends full power to the coil to fire the plugs as the normal resistor wire drops the voltage too low. Either the ignition switch , the large plug in the fire wall that feeds the starter or any other wire along the path is bad.
My guess is that your firewall plug is bad as I had problems with the same plug on my 62 Chevy II. They get corroded.
Good Luck
Luis
