Sounds like beefing up your engine grounds would be the best place to start. Running a lead from battery - to one of the mounting bolts for the starter and including an internal/external star washer will ensure maximum flow through the starter.
The Motorcraft type starter is designed to be functional with only a single + lead with grounding through the starter body. This design works okay but can work better by adding a - lead of equal or greater gauge than the +.
If your starter has phantom run on issues I would look at the solenoid as it is the only thing that switches + power to the starter via a key on power source. If you have it wired up so + can sneak back to the S terminal, an internal fault will engage the starter.
-= Francis =-
These, in the day when heaven was falling, The hour when earth's foundations fled, Followed their mercenary calling, And took their wages, and are dead.
Their shoulders held the sky suspended; They stood, and earth's foundations stay; What God abandoned, these defended, And saved the sum of things for pay.