What is the difference between a starter for a 307 Chevy and a 350 Chevy?

Some starters are intended for a 153 tooth flywheel, some for a 168 tooth flywheel. Most 307's had 153's, and most 350's had 168's, but you could put a 153 on a 350 or a 168 one a 307. Both engines are internally balanced, so as long as you have the correct starter for the flywheel, it will work.

The one other difference between Chevy starters, is the length of the armature. The longer one is heavy duty, the shorter one is standard duty. Either will work as long as it's mated with the right flywheel.