75 duster slant six weak spark

yes the resistance in run matters. Todays ignition modules will have only 4 contact posts under that connector. They operate on full 12 volts so they don't use the second part of dual resistors. Anyway... With the switch in run position the coil should receive about 9.5 volts. That is 12 volts going through the 1.5 resistor to get there.
In start the coil should get all the 12 volts.