Brotherhayes...

You may want to reconsider. When I talked to the pond drain people about my pond they told me I MIGHT be able to make do with a 6 inch pipe if I used a syphon system. My pond will be two acres with only 30 acres of watershed. You are going to be having a whole lot more drainage than me. I would keep the drain pipe you currently have and then look at adding one or more syphon drains. They drain water MUCH more quickly than a stand pipe and you don't need to worry about cutting through the core of your dam to get one installed.

Water dries, rocks crumble, and trees die. The only thing that is eternal is the reputation we leave behind.
- Ancient Viking Proverb