Pat, need a better idea of what your overflow pipe is going to look like, if its actually going to using a siphon by just drilling a small hole (1/8") in the overflow pipe at the lowest level you'd want the water to be able to fall to, once the water level falls below that point air will be introduced in and automatically break the siphon for you. I think you mentioned your tank is drilled at the bottom, though, if the pipe stands striaght up in more of a 'durso standpipe' configuration it will automatically break once the water level drops to a certain level, or you can adjust it by drilling a hole where you want it to stop.

Just be sure to take into account how much extra water capactity your sump will have to hold in the event of a failure.

Note Durostandpipe looks like this. Image borrowed and copyright of http://www.dursostandpipes.com/