OK, I have gone around and around with this. GMC truck, Chev 350, TH-400, offset-bolt stock starter. Getting horrible grinding noises and no cranking of the engine. Tried various combinations of shims, very little change. Loosened the bolts and wedged a wood block between the starter and oil pan, tighten bolts back up, works perfectly! Drove around all day, started the truck about a dozen times and progressively getting noisier (but still starting OK). I can tell this is heading back to square one, so needing some insight on how to solve this once and for all. Is there some sort of bracket that I'm missing?? I hate having that block of wood crammed in there, but it's doing the trick for now (but it's not "right" so it will bother me).
Thanks,
Chris

Are you using the correct bolts? (they are knurled) Are they bent? Regular bolts will allow the starter to piviot slightly causing problems.

so needing some insight on how to solve this once and for all.
Replace the starter and the bolts.

Hmmm...my bolts are not knurled! Will look into that tomorrow. The starter itself is fine, I'm quite sure......has been working great with the wooden block wedged in there. Thanks for the bolt info!
Chris