If your engine has ANYTHING extra wired to the coil's positive post, the extra load will act as an automatic rev-limiter. Back in the day, owners used to wire their Holley electric chokes to that 'convenient' point of 12V, and the engine then would consistently misfire or quit at 4200-4500 rpms.
Since you already have a Duraspark, you can get even more protection by using a Ford solid state TFI coil from the next iteration of factory electronic ignitions. AutoZone carries them cheap. That transformer-type coil as designed takes a special molded wiring plug, but inside the coil's molded connection are a pair of std male spade-lugs clearly marked + and -.
Just add female spade lugs to your stock coil wires, plug them into the TFI coil correctly and you're set, almost for life. You might want to rotate the TFI coil mount 90 degrees so its molded electrical connection does not act like a water container if you drive in heavy rain. Now, leaving the ignition switch 'on' for more than a few minutes with the engine off will NOT burn out your old can-of-oil coil or the ignition wiring. There are more li'l tweeks for Ford ignitions, too.