Charlie, when you removed the starter, were both the mounting bolts tight, and were you able to torque them both down when you put the starter back in? There's a reason I'm asking.
The mounting boss for the outer starter bolt on GM small blocks is a weak point. Though it isn't common, they've been known to break, resulting in the starter being able to shift position when it tries to engage the flywheel. It doesn't fully engage the flywheel and just spins. Again, it isn't a common problem. Just mentioning it because you can't seem to find another explanation.
