no spark from coil

A big change in voltage with a change in ECU is a little troubling

This is because the Mopar ECU DRAWS COIL CURRENT with key on/ engine not running

Here's what this means

Your current path is battery....through harness.....ignition switch....through harness.....to ECU and ballast resistor......through the coil.......back to the ECU (coil NEG) and to ground.

In other words you have a current path through the coil and ECU

WHAT THIS SHOULD DO is cause you to read "close to battery" voltage at the "switch" end of the ballast but much LOWER voltage at coil positive after it has dropped through the ballast.

After the voltage has dropped through the coil, the ECU being the ground for the coil NEG.....the coil NEG terminal should read almost nothing.

You sure you grounded the new ECU?

Could be distributor pickup, etc, but that's hard to troubleshoot, other than checking reluctor gap, then just swapping either a different distributor or new pickup coil, and changing the pickup coil is no fun "on the side of the road."

If you can afford it, I'd buy a rebuilt distributor. Any electronic distributor from 72 later that has a vacuum advance for a small block

If you run a tach, disconnect it for now