NOTE: After shiming the starter correctly I always put that mount that everybody tosses or is missing on the end of the starter that holds the starter square and keeps in from shifting.
You mentioned that after shiming the starter and cranking the engine a few times the starter is grinding again, well whats happening is that the starter is moving out of position horizontially which is what the rear support bracket is there to prevent. if you dont have one you can fab one up out of some 1/8 inch angle iron and your grinding problems will go away.
Goodluck >< Cp