I have a 63 dodge pickup with a habit of burning out ignition coils within 2 weeks. It’s happened with the slant 6 motor, it’s happened with the 318 v8 and its also happened with the current motor a 440. I have changed the ballast resistor, and the ignition switch, as well as the starter solenoid, and all related wiring.
Check the voltage going to the coil while the engine is running and make sure the ballast resistor is doing its’ job. I would guess the voltage should drop down to around 10 volts.
Look to see if someone has bypassed the ballast resistor. If I recall correctly, the ballast resistor is only used during the start cycle where the slow turning of the motor allows excess current to flow through the coil. Once it is started, the points interrupt the circuit enough to control the current flow.
If you have replaced the distributor with an electronic unit, then you need a different coil and ballast resistor because the electronic distributor has zero dwell time.
Are the coils overheating to failure or are they having an internal dielectric failure? Have you opened up a failed coil to attempt to see what is actually happening? Ignition coils as I recall were quite durable. The only failed coil that I ever had was due to the use of a homemade electronic capacitive discharge unit that was intended to make a hotter spark and to extend ignition point life. Is your supplier furnishing coils intended for a 12 volt and not a 6 volt system? Defective coils are not an impossibility. Hopefully your vendor is still up to speed with making these correctly.
The ballast resistor is bypassed during the START mode so the coil has more energy for firing the plugs. Once the engine is running then the ballast resistor is used to cut the voltage to the coil since less power is needed to fire the cylinders.