Starting spark frequency? (ignition issues...)

(Long story, question at the end)

I have a 73 Duster with a 340 and an electronic ignition. Three weeks or so ago the car started and drove fine. Tuesday I went to drive it and it did not start. I checked for spark at the #1 spark plug with a spark tester and didn't see any spark. Then I checked all of the voltages I found recommended in other posts with the key in the run position. I had 12 volts on the ignition wires (there is a dual ballast resistor, but it's been wired to so that the bottom and top resistors have been bypassed), 12 volts on the positive side of the coil and 12 volts on the negative side of the coil. I checked continuity between the ECU harness and the locations on ballast connectors, distributor connectors and coil connectors and everything checked out. The 12 V on the negative side of the coil led me to believe that the ECU might be bad and, since I didn't have a backup anyway, I ordered one.

The original ECU was a REV-N-NATOR (in the process I found these are out of stock) and I replaced it with Carquest premium module. After I installed it, the car still did not start, but I measured and now have 1-2V on the negative side of the coil. Somewhere in the process I read to check the AC volts coming out of the distributor and read 0.2-0.3 VAC which seemed a good bit lower than the 1VAC mentioned in other posts. So I thought maybe the pickup had shifted or gone bad so today I took out the distributor (a firecore model from the RT garage) and the air gap seemed fine if I was using my feeler gauge correctly. I spun it by hand and never read above 0.5 VAC on my meter. I also have an older MP distributor and it measured upwards of 0.8-0.9 VAC with a gap > 0.010". I adjusted that one and, spinning by hand, got voltages greater than 1 VAC, so I decided to put the MP unit in the car.

I got everything hooked back up and the car still wouldn't start. I did check for spark at the coil at the distributor side of the HT wire (I had to buy another tester since my other one didn't seem to be configured for this wire) and got a spark. We tried a few more times to start it and when we backed off the key the engine would spin backwards for a second or so. This clearly wasn't right and figured I either messed up when I put the distributor in or cross wired something. I rotated the engine back to about 15 degrees BTDC, reset the distributor cap and hooked the wires back up. After this we tried to start the car. The first time was nothing. The second time the car fired right up and idled fine.

We let it run for a while and I checked the timing. Everything was going fine so we shut it off and I plugged the vacuum advance so I could check the mechanical advance and the car would not restart. We tried multiple times and it would never restart (it never ran backwards during this evolution so that was an improvement). I still had the timing light connected and it would occasionally flash, but it seemed to take a good bit of time for the flashing to occur. But I have no reference point for how often it should spark when the engine is being turned by the starter.

How often should the coil spark during starting? It seemed to take five or more seconds for the coil to fire a spark and 30 or more seconds to fire at the #1 cylinder as indicated by the timing light. This just seemed long to me. Maybe the distributor still isn't sending a consistent signal to the ECU?

What should I do next to troubleshoot?

Thanks for all your help.