The wiring issue seems to stem from using a II or III. The Petronix 1 is compatible with the Ford Tach.
The simplest way to explain it is that the Petronix needs 12v run directly to it. The Ford yellow top will self destruct with that.
The simplest place to pick up a 12volt switched source or the Petronix is at the input to the tachometer inside the dash. You can get access to that by removing the radio and the ash tray assembly.
There is where you create a "y" splice. One going back into the tach input and the other to the red input wire into the Petronix. Don't run that wire to the plus terminal on the coil as the Petronix instructions say.
The plus terminal of the coil is going to only take two wires connected to it. One is the pink resistance wire running from the tach output wire. The other is the 12v red wire running from the starter solenoid I terminal.
The I terminal from the solenoid supplies 12 volts to the coil while the starter switch is in the "start" position, turning over the starter motor.
When the ignition switch is in the run position, and the engine running, the voltage reading at the plus terminal on the coil will be 6volts. If it is more, then replace the resistance wire. They can wear out with time.
The entire issue really is caused by the Ford tach that reads off of the plus terminal of the coil. Just about every other tach system on the planet reads off of the negative terminal.
This just has to do with the original Ford engineering design of essentially just "splicing in" the tach into the electrical system of the car to reduce major wiring redesign of the standard ignition system.
We have to live with that peculiarity.
I don't have a Petronix II or III, but am informed by those that do that those are the ones that need a wiring adapter to make the tach function. I personally can't confirm that.
In all honesty, there isn't much advantage to using a Petronix v. stock points. It does give you better dwell characteristics but there isn't much advantage to that. Even the race Cobras and GT40's used points and yellow top coils.
A standard set of points should easily give you 30,000 miles before needing replacement which is a very simple thing to do even on the road if need be.
People are carrying spare Petronix units with them. Those two factors should suggest something to you right there?