I can't test the ECU but I have a good one coming from Hoppy. All the voltages and resistances look good except the cranking voltage. I wish I had a Sun Vat 40 so I could do an amp draw on that starter. A guy on B bodies changed to a new style Nippon Denso starter and it cured his problem. It spun it over faster but with less voltage loss.
I am still running the starter from a Dakota I bought for 40 bucks , it's first 7 to 8 years on my 170 slant, now on my 360, I will never go back to the large slow voltage dragging starter