Is the 6" overflow pipe something you can directly connect to another pipe? (For example, is it clean PVC or 70 year-old rotting galvanized steel?) You can certainly use your existing pipe if you can make tight connections. However, air leaks are the enemy of siphons.
If you can't do it that way, then over the dam with flexible corrugated pipe is pretty cheap and easy.
Here is a link to a video. They aren't rocket scientists, but do show all of the key steps.
Pond Siphon
