I spotted your PVC overflow pipe right away. I want to do something similar. Our land is totally flat, but there is a highway drainage swale about 40' away that is lower than the pond. I don't have to worry about a spillway (if the pond overflows, it will just flood the land, but if there's that much water, the land will already be flooded /forums/images/graemlins/crazy.gif).
I plan to keep the pond topped up to a more or less constant level by pumping water from the well, activated by a float switch. I want to put the outlet pipe a couple of inches higher than that level in case we get a gullywasher storm.
What size pipe did you use? Did you screen it in any fashion to keep floating leaves, etc. from clogging it?