One thing comes to mind, in general. Although some high performance ignition coils run on full battery voltage, Ford oem ignition coils in First Generation Mustangs run at a reduced voltage (4.5 - 6.9 volts per Ford Shop Manual), to include the oem ignition coils on the 71-73 Mustangs and Cougars (and nearly all other oem primary ignition systems back then). If you are running 12 volts (or full battery voltage) in the Ignition Switch "Run" position, but using a coil designed for reduced voltage operation, you could be overheating the coil, and causing it to fail before it would have failed otherwise.
I have heard plenty of folks tout the benefits of running their coils (oem and otherwise) at full battery voltage to "get a hotter spark." I prefer to not say, "nonsense," but it is tempting. Yes, running 12 (13.2 or so, actually) with no resistor wire or ballast resistor will indeed increase the potential output of the secondary circuit from the coil. But, it really is not needed as the oem ignition coils are designed to run at about 4.5 - 6.9 volts (per Ford Shop manual, I thought it was more like 8 - 10 volts), depending on the year and model vehicle.
"Yeah, but when you are starting the engine you are bypassing the resistor wire circuit and sending a full 12 volts to the coil, so that has to be better," - so sayeth the armchair experts. Well, yes and no. Yes, on those older ignition systems, when cranking the engine over the primary ignition circuit's resistor is bypassed and full battery voltage is provided. But, when cranking the engine over the battery voltage drops to about 9.6 to 10.5 volts, so although the ignition coil is getting full battery voltage, that voltage is only about 9.6 - 10.5 volts - NOT 12 volts as assumed by so many folks who never really think about how the primary ignition bypass really works.
Also, if the secondary ignition system only requires 14,000 volts to jump a spark plug gap, that is all it will put out, regardless of how much it can potentially produce at maximum. That is because once the voltage needed to jump the plug gap is achieved, the current is going to jump that gap and any added power will be used to sustain the length of time the "spark line" exists. So, even though using a coil that boasts produces 30k voltage, it will not put that out if all you need is 14k volts to light off the spark plugs. And, in fact, a higher voltage ignition coil may begin to cause otherwise avoidable problems in the event a huge amount of voltage is needed to let the high voltage current find ground, whether jumping a spark plug gap or finding ground elsewhere due to weak(ening) spark plug wire insulation, weaken(ing) spark plug boots, etc., or other places (cracked spark plug ceramic insulator).
So, back to the larger point. I suggest you research the primary voltage needed for the ignition coil you are using. If it is designed to run at 4.5 - 6.9 or so reduced voltage, make certain you are using an appropriate resistor wire or ballast resistor ignition circuit. If it does not need a resistor, and is designed to run at 12 (up to 13.2 or so) volts, then be sure to use a circuit that provides full battery voltage with sufficient amperage available to properly saturate the coil when current is sent to it (when the points are closed). But, I hope you do not fall into the trap of thinking if reduced voltage is good, full voltage must be better. Voltage at too high a level will shorten the life of the ignition coil, at the least.
My two cents. Rant over....
