Ifishnow,
In a similar situation last summer, I trenched, with my backhoe, through the spillway. After draining most water, then cleaned out what I could and pushed the remaining muck in to form a submerged island in the middle of the pond. Then, raised the dam, or at least tried to raise the dam until I got caught by fall rains. Will finish the dam this spring. Raising the dam is much cheaper than excavating....and the island provides great structure. Also, I would consider just going with natural spillways and forget the costly pipe.
