76 Dodge dart wont start

Yes, I read of many intermittent problems with the Mopar ignition module. Some people claim no problems ever, but most owners keep a spare in the glovebox. Of course, the problem is often not the module, but the wiring and connectors, especially the bulkhead connector.

As you found, intermittent problems are the hardest to resolve. You saw a spark with a plug off, but seemed like no spark later. That is why I recommend an in-line spark tester. My Newport died suddenly last spring, with my wife and daughter in the car, so under pressure. It has a Crane XR700 ignition. Put an in-line tester ($3 Harbor Freight) and no flashes from the neon bulb. Swapped another Crane box I had in the trunk, good flashes and engine fired. I left the tester in-line and drove home.

You changed to an HEI module and it fired right away. Was it actually the module or just from wiggling the wires? If a bulkhead connector problem it will be back. Regardless, keep backup parts and diagnostic tools in the trunk and your life will be simpler.