Duster won't fire in cranking position.

It helps to understand "how this works." Regardless of whether you have a breaker points or Mopar breakerless ignition, or whether you have a 2 or 4 pin ballast, the one side of the coil resistor that goes to the coil is all wired the same.

One one end of the resistor is power coming from the switch in "run" called IGN1. IGN1 also branches off some place and feeds the voltage regulator, the alternator field (blue) and some smog du dads depending on year. On the other end which goes to coil+ is power in start which is the bypass circuit called IGN2.

IGN1 IS ONLY HOT during "run." It goes dead in "crank."
IGN2 IS ONLY HOT during "crank". It is not present in run. That voltage comes from a completely separate contact on the igntion switch, is brown most years, but varies

So use a light or meter and check coil + for voltage during cranking. It should be same as battery. If not, "back up" and follow it back (diagram) through the bulkhead to the ignition switch, and check there at the switch, at the connector coming out of the column.

You can download simplified diagrams at MyMopar as well as service manuals, free