Did the starters you installed have the 1/4" quick disconnect on the solenoid, or did they have the post?
Seems Ford had Some Problems with that Q-D and reverted to a post on the starter solenoid and a ring terminal on the wire. For easily seven years I repeatedly had to dive under the right side of my '94 and move that Q-D around to scrape the crud off it so I could start the truck -- either that or park it on a grade so I could get it rolling fast enough to pop the clutch in 2nd and start it. I finally had to replace the starter when the 1/4" terminal on the starter solenoid built up enough crud that I couldn't scrape it off with my pocketknife and restore contact. That's exactly what it sounded like -- an emphatic "KLIK!" instead of the sounds of a V8 turning fast enough to start.