I had a similar issue I bought a 79 F250 that I'm pretty sure they left sit after they couldn't figure it out. It also had a new starter on it that did appear to fit properly except it would grind at times make a awful noise while running. I pulled the motor out just to double check everything and what I found is the wrong starter was used. I may be wrong but from what I gather it's a 351W starter. I'm guessing the parts stores just saw 351. The nose cone is turned too close and the teeth were chewing on it. I picked up a used one and held them side by side very easy to see the difference. Works great now.

 

4 year old thread here that's been revived