1990 GMC, will not start. Spark makes no sense to me

Helping a friend with a 1990 GMC with 4.3, 6 cylinder. He saw no spark so after checking on prices for things like rotor, cap, module, he just bought a rebuilt distributor. Installed it and it still will not start. If we run the coil wire to a plug, we get a spark. Check an individual plug, no spark. Yes, the rotor is installed ! I would think module, but rebuilt distributor came with one. Crank position sensor? I don’t see one. Does a 90 year have one? How bout ECM?
Any ideas would be great. Oh, when it stopped working, he was driving it and it spit and sputtered and stopped. Now it would even cough a little. HELP !
And, just in case…where is the ECM on this truck?

If you have spark at the coil but not at the plug, then it should come down to three possibilities. Cap, rotor, wires. This engine did not have a crank sensor by the way. The ECM is located on top of the heater case on the right side if I remember correctly.

Triple check the position of the distributor. I got one off by one tooth once. (Assumes the driven gear comes out with the unit.)

I agree with @"pete peters"‌. You got spark at the coil. The spark is finding ground before it gets to the wires, which means a bad cap, rotor, or spark wires. Look inside the cap and under the rotor for carbon tracking.

Just an update… It was the ignition module. The one that was in there was only a month old !
I’m thinking there might be something going on with the coil and it is sending hi voltage back to the module and making it fail.