I wonder if that thin wall plastic pipe can withstand the forces of a full siphon effect if you extend the pipe down the back of the dam. You may not want to install an anti-vortex plate unless the manufacturer can verify it won't collapse.
That's a good point... I am still leaning toward just using existing large stone stacked wider and then fill all of the gaps with fast setting quickrete to smooth it out into a tialrace. With the sides maybe 6-8" higher than the middle.
