I like doable, gives me a shot at making it work.

Current plan is for 8" pvc riser pipe going down to a 'T' which incorporates a valve on one side and an exit 6" pipe out the other. This 'T' will be a few feet above pond bottom. I have a spot picked out for emergency spillway but may need to take down a few trees to get it wider. I don't know how wide it should be for this pond. The spot I have picked will run slightly downhill (I'm thinking for 30') then increase more downhill.

I'll need to read up on the siphon system, but contractor and I didn't discuss it. What is advantage? I guess dam slope will be 3:1 both sides. I'm thinking that's standard minimum, but it will depend on how much material we can accumulate from on site excavation. Might be tough to get that much out of existing grade.

I have couple of questions:

What would be a good bank slope if water level varies some years?

For my narrow end, what would be good shallow depth that wouldn't promote weed growth and help fish that like shallow areas. I'm thinking it would extend maybe 50 feet into pond then increase in depth. Am saying that without any research though.