Well, never having had to do that, I wonder why it became necessary unless the fittment is that bad.

 

i think i might try another starter to see if it fits any different.

 

Ever notice how the starter drive gear on a factory GM starter is almost 3 times as thick as the ring gear? It's because the production tolerances on starter bolt holes are THAT bad. This is the same reason GM starters have to be shimmed away from the block. They made the drive gear extra thick to compensate for not knowing how far out it would need to extend, and you shim it to compensate for not knowing how far from centerline the boltholes are. You may just have a block that's on the extreme edge of factory tolerance. Try a different starter and if it has the same problem you'll just have to space it back farther.