1988 ford f 150 300ci straight 6 ............i have put in 3 new starters and i am still getting grinding ive shimmed it out several different ways and also made sure it wasnt a starter for a standard ..... what the hell is wrong

Optional Information:
1988 Ford f-150 300ci 6cyl.

Already Tried:
3 different starter and shims in different configurations/p>