If the resistor goes bad it can only open. Then you would have zero volts in the on (run) position.
You say 7 Volts at the coil with key in the run position so the resistor wire is OK.
I would change the Ignition Module, sounds like it is holding the TACH side of the coil at ground all the time.
Does the TACH side flash a test light while cranking?
Not sure about your truck but the ballast resistor wire on a tuck in the 80's look like this.