[big455]

does overfilling a th 350 automatic damage any parts, or how does it effect drivability?

[adtkart]

Overfilling any transmission or engine does the same thing. It causes the fluid/oil to become full of bubbles, causing less lubrication. This is because the fluid/oil is being whipped by the revolving parts. With an automatic transmission it also reduces the actual fluid pressure because air is compressable, where fluid isn't. The correct level is actually just below the revolving parts to avoid this problem.