'75 Duster rewiring - opinions on direction?

Cheapest and easiest thing to do is plug in a factory harness to make all of your standard connections (lights, wipers, whatever factory gauges you intend to use, heater, etc) and then create a sub harness for all of the aftermarket stuff.
You'll be miles ahead when it comes to troubleshooting future problems.
I have a very nice 73 Harness, which is much simpler than the 74-up one, and can handle all of the cars stock systems.
$150 plus shipping if you're interested.