Carbon tracking through rotor. More HEI woes.

Many good comments, there are electrical measurements that can help.
1. Ohm out plugs, cables and cap from plug tip to cap inner terminal. Typically a few 1000 Ohms, it varies with length. One open cable can create your problem.
2. Use a scope with high voltage probe (2Kv) to monitor coil plus. The secondary voltage is approximately coil turns ratio times peak coil primary voltage. Typical peaks 300 to 500V. So with 100 ratio, 40kV at 400V. Use time base to look at one in detail, 8 for consistency. When high peak strikes over plug, voltage goes low, and the decays, those should look similar too. Plug wires are part of voltage drop too.
3. Monitor coil current with scope by inserting 0.1 Ohm 10W NH (non inductive) in series with coil, use scope on resistor terminals, using suitable isolated input scope or 2 channels in differential mode. Wave form will show coil charge time (varies by tyipical a few ms) and peak current. The current measured is 10x voltage, so 0.5V peak is 5A.
Typical peak currents reach 3-5A for Mopar electronic, but up to 7A for GM HEI One solution is to add ballast resistor as necessary to bring current down. That will also bring down secondary voltage. With a well tuned engine 20kV, is often enough. To much or too little fuel is not best fixed with ignition.