Wow, that's a big pond Canepole! :p The green is for "envy".
One way is to run the overflow pipe through the dam, then elbow it straight down close to the bottom of the pond.
The pipe must run through the dam at the maximum water level you want. A small vent pipe MUST be tapped into the main horizontal pipe, sticking straight up. The vent pipe should be taller than the water should ever reach, and have a tee on top with hardware cloth to prevent critters from blocking it.
Leaks that follow the overflow pipe are not a big worry with this type of overflow.
