Starter Problems!
First thing you need to figure out is the flywheel. It should be turning in a smooth circle, if it is turning and appears to be more in an oblong circle, something is wrong. This could cause the flywheel to be too close to the starter at one point, and too far at another. You can check by turning the motor over by hand while under it and measure from the teeth of the flywheel to the snout of the starter or something.
Another thing you can do is disconnect the starter and apply 12 volts to the SOLENOID ONLY to make it pop the gear out to engage the flywheel. DON'T connect the actual main starter motor wire or it will try to turn the motor over. You can measure your contact at various points doing that. When you check with the gear engaged, take a small size paper clip and stick it in the teeth where they mesh. You should be able to get the clip in, but not have a lot of extra room.
Also make sure you have the correct, right lenght starter bolts. The knurled part needs to engage the starter body by the block to help keep the bolt to motor fitment tight. If you put washers under the heads of the bolts, the knurled part may not fully make it into the starter body allowing a little movement.
Finally, I had luck on my starter by just shimming the outside bolt. I had to cut a shim in half and just put it under one bolt.
Good luck with everything. Hopefully this helps you some.

