Midwest -
I am not 100% sure that this is your problem, but Ford used a number of different size and tooth number flywheels on their light duty truck. Our Mustang t5's used a 10 or 10.5 inch clutch but always with a 157 tooth flywheel. Ford light duty trucks used often an 11 inch clutch and a 164 tooth flywheel.
From what I understand, the starter for the two tooth counts are only SOMETIMES interchangable.Way back they even had one that was 147 teeth.
You don't really say what it is that you replaced 3x, the starter or the flywheel. Assuming that you have inspected the flywheel for cracks (very carefully) and your started went bad, you may be replacing it with the wrong starter. If you are changing the flywheel, you may be using the wrong flywheel.
Another thing to keep in mind if you have changed the flywheel. It is very tempting for the parts counter to hand you a flywheel that indicates it fits the whole small block family including 351W. There is no problem with this prior to 1981. After 81 the 302 crank was changed to a different imbalance while the 351 was not. You can if not carefull have the parts guy hand you a flywheel that he thinks is correct and will bolt right on but will be mismatched and shake your bearings to pieces.
If that doesn't help. Please let me know what it is you have changed 3x.....the starter, the flywheel or both.
Good luck | 
I need help with motor problem 
