340 Not firing on all cylinders

You have a 2 terminal resistor/ 4 terminal box?

"Run" at bottom right represents your original "ignition run" or IGN1 wire, traditionally blue, but different color in later years

"Run" is hot ONLY in run and goes "cold" in start, so no power from that line during cranking

"Start" at top right represents the brown "bypass" circuit or IGN2. This is hot ONLY in start, and is the ONLY source of power in starting

Wiggle and push / pull the connectors at ballast, distributor, and ECU alll to scrub the terminals clean and feel for tightness.

Check the reluctor gap with a brass feeler for .008", that's inches not metric. Inspect the reluctor/ pickup for strike damage and debri, rust, junk, and feel for shaft side play Hook your mulimeter to the two distributor terminals on low AC volts and crank the engine. The distributor should generate about 1 volt AC

Make ABSOLUTELY certain that the box is grounded. Scrape the box ears clean, and around the bolt holes on the firewall, and use star washers.

If you have not, try 2 clip leads---use one to bypass the ballast resistor, and use the other to hook 12V directly to the coil + terminal. Do not leave it hooked up for more than long enough to test, and watch the coil for heating.

Check coil voltage WITHOUT the coil wire. Instead, use a grounded probe such as a clip lead and screwdriver, held near the top of the coil HV tower.