[mild68ss]

I just went through the same problem with the starter grinding and making a horrible metal to metal sound. Actually wore the teeth on the flex plate.
I tried shims, more shims less shims. I tried a new flex plate too. Same thing.
I realized from another post that the starter was too far away from the flex plate. The shaft of the starter should be about an 1/8 th of an inch from the flex plate teeth. Mine was too far away and I think it wasn't staying engaged in the flex plate teeth long enough to get the other starter teeth to match up. I cut three shims in half and used them on the outter side of the starter and all is great.
It really puzzled me because I used the same motor, flex plate, starter and torque convertor for two years.

Do you have starter bolts with a knurl?