what voltage should I have at the coil

I didn't quite understand how the spark tester is hooked up. Best if one side is to the HT wire on the coil and the other side grounded to BATT-. That takes the distributor out of the spark path. If you still hear a clicking but see no spark, the coil might be arcing internally (bad). I recently had that with a coil pack in my 3.8L and changing it fixed my mis-fires.

Since deep into it. Remove the distributor (mark rotor & housing position first) and spin it by hand. You might need to clip the body to ground. Without engine cranking noises, you can better hear any sparking or arcing.

Nobody suggested checking the reluctor gap in the distributor. Should be 8 mil max and best to use a brass feeler gage. It sounds like you are getting a magnetic signal, but your multimeter isn't exact. The pickups are cheap. I paid ~$7 for a new one for my small block (could be same PN).

Clip a ground wire to the body of the ECU. Indeed, I would just run a permanent ground wire. I never rely on a rusty sheet-metal screw like the factory did.

Finally, let us know what fixed it. Too many fly-bys here, or people who say, "sold the vehicle, that fixed it".