Not getting 12v to coil on start ('73 Dodge W200 360La)

Sounds like you mis wired something in the conversion back and forth

The basics.

When IGN sw is in "run" the IGN1 / run wire feeds 12 v to one side of the dual ballast. That end has both terminals jumpered together. Through the ballast we go, to the coil, and to ground through the ECU box which DRAWS CURRENT ANYTIME the key is in "run." (Current flow is actually backwards, neg to pos) The current through that circuit causes VOLTAGE DROP through the resistor, so the coil DOES NOT see 12v. Rather, engine off, key on, coil + should show somewhere between 6-10V. This varies with resistor, with coil type, etc

With key in "start" IGN1/ run GOES DEAD. Starting power is now supplied ONLY by the bypass circuit, often brown, IGN2, goes to the coil + side of the ballast resistor. This circuit feeds "hot" power to the coil and goes BACKWARDS through the coil resistor to feed back power into the ECU to power it during cranking

Coil + should show "same as battery" during cranking, and not below 10V absolute