Ford tachs are current sensing. You also do't get 12V to the coil EXCEPT when you are engauging the starter. After that it drops to about 6-7V. If you actually read directions, the Intructions tell you that you must have 12v constant. You can avoid digging through your wiring and unplugging the resistor wire, which is VERY difficult to do on 67 and later vehicles. Simply pull a wire a 14 gauge wire though the grommet to the positive wire on the Pertronix. The wire can either be ran to the ignition switch 12V ign constant hot, or the red wire that goes to the tach. The black wire from the Pertronix goes to the negative side of the coil. Use the nut. Slid the black Ford wire that comes from the harness onto the negative post of the coil. The failures that many blame Pertronix for will be an issue of the past.
The II and III version are absolutely dependent on 12V constant. Many get away with running the Pertonix I for years sometimes.
I have the Pertonix II on my Shelby. It's been there 14 years, never an issue. I like the two because it automatically adjust the dwell.
-Keith