In stock configurations, the "I" terminal on the solenoid supplies a full 12V to the ignition system only while the engine is cranking. When you release the ignition key to the "run" position after engine start up, the solenoid is disengaged (starter stops cranking), and the 12V at the "I" terminal is also disengaged. This results in reduced voltage measured at the "I" terminal because it is now being fed only through the resistance wire (pink) circuit. To install the Duraspark ignition box, you need a switched 12V supply to the "red" wire on the Duraspark box. On my 66 Fastback, I disconnected the PINK wire at the ignition switch (red-green ignition switch wire) and plugged in a new wire (non-resistance wire) to feed the Duraspark box. I didn't use the white wire and have had no issues with start up; however, I have a high-torque starter.
The coil is another issue to consider. As far as I know, Ford Duraspark coils required a resistance wire to reduce the 12V voltage after startup. I know this is true on my 85 Mustang GT with Duraspark. So, unless you are running a coil specifically designed for 12V (non-Ford), it will need a resistance feed. I'm using an MSD 12V coil on the Fastback so I didn't have this problem. Also, I think that some of the later TFI Ford coils (after Duraspark) had a different look/construction and can take a full 12V.
FYI, I'm using an MSD billet distributor and drive it with the Duraspark box. The mag pickup in the MSD is the same pickup that's used in Duraspark distributors. I had some issues with the MSD 6AL box and installed the Duraspark box as a back up in case of problems. I can switch between the two boxes and have not been able to tell the difference in engine performance (by the seat of the pants).