You might indeed be better off using the better harness, and adding the Duraspark to that. I think it's just three wires shy of plug and play:
There's the resistor wire to the coil, and you can almost certainly get away with using the resistor wire from the points coil and splicing the DSII horse shoe connector to that.

Then I think there are two or maybe three wires to the DS II module/box:
There's a 12V+ input, which comes from the "I" terminal on the fender mounted starter relay/solenoid. (It's used during starter operation/cranking, to send a full 12 volts to the coil.) It's important to use the "I" terminal, so if your relay/solenoid doesn't have it, you'll need to pick one up. Don't be tempted to source that voltage from the "S" terminal, because it caused an occasional starter run-on condition on my old '68 Cougar. When I switched to the "I" terminal, it never hung up the starter again.

And I think there is a 12V+ input to the module/box for when the ignition switch is switched back to the "Run" position.

There's probably a ground wire too.



I ended up using the 1988 harness in my Mustang when I converted it to Mass Air, and just adding the wires for the MAF sensor and a few other inputs, even though I had a whole '92 Mustang to cannibalize... Too many connectors weren't the same, so it was better, and less work, to do it the way I did. I was intent on swapping the whole EEC harness until I did some research...