Also scrape the firewall and mounting ears of the ECU and make sure they are tight.
"Work" the ECU cable connector in/ out several times to scrub the terminals and to "feel" for tightness. Inspect them with a flashlight for corrosion
Did you read the coil + when cranking or with key on/ engine stopped? This creates a different circuit.
With key in "run" and engine stopped, you should measure perhaps 6--10V at coil + but when cranking USING THE KEY, the coil + should read very close to "same as battery".
Don't depend on your spark test if you have resistor wires. Always "rig" a real wire out of the coil tower to test spark, and it should be hot, blue, and at least 3/8 and often 1/2"
Other voltage issues to check is the ignition "run" (IGN1) line through the bulkhead connector, through the ignition switch connector AND THE switch itself.
One way around voltage drop here is to cut the "run" wire soon after it enters the engine bay, use the firewall end to trigger a Bosch relay, and feed the relay contacts off the starter relay. Connect the engine bay end of the cut "run" wire to the switched contact
If voltage drop is the problem THIS WILL also prevent overcharge voltage as well
