Livewell over flow plumbing.

That's how I did it. I used 1 1/4" PVC straight back under my false floor but my coffin box has a bulkhead to the rear of the baitwell for pumps. I put a PVC shut off valve right behind the bulkhead so I could use it as a recirculating well or aerated. If that doesn't make sense I'll post pics but my boat is upside down being painted until tomorrow. Since water is self leveling just make sure your thru-hull is lower than the top of your overflow tube.

 

When I was building a livewell for my offshore boat,
I was told that the diameter of the overflow pipe
should be at least twice the diameter of the input pipe.

 

I was told the same thing as you Brett about the overflow being twice the diameter but my intake is 3/4" so I figured 1 1/4" was close enough.  I have a 500 gph pump as well so if it doesn't drain fast enough I can just cut the tube and let it drain into my bilge but I'm not too fond of that option for a few reasons. 

I'll let you know how my setup works because I have been wondering the same thing as you as far as the difference in the height of the tube and the thru-hull affecting the efficiency of discharging water.

...as a matter of fact I'm going to do some science experiments in the garage right now.  results coming soon.

 

Firecat, did you have a valve you could partially close on your inlet? If you have one you can always restrict inflow so that you can't overwhelm your drain.

 

Took me a while to respond but the weather was too unpredictable to paint for a while there.

I got the whole thing rigged and tested it on the water and it worked flawless. 500 gph pump draining into a 1 1/4" pipe was all I needed. My discharge is below the waterline so I don't know if the tension already on the water helps it drain smoother. The difference between the top of the drain tube and the thru-hull is about 14-16 inches. I'll get some pics soon but my boats at my brothers house and my camera was dead when I went over there yesterday.

 

Drain it thru the floor, out the bottom.

My livewell is in the rear deck, but far forward of the transom, and not much different than yours in the center console area.

I used the 2:1 rule, the drain is only gravity fed, and is 1.5" diameter. The fill (although not yet plumbed) will be 3/4".

In my case the overflow drain also doubles as the tunnel vent, but you could easily use it as a drain only. If you angled the thru-hull in the hull bottom at 45* or better, it wouldn't get any backflow when running. There are many things you could do here, but it's not an issue in any case.

At rest, the thru-hull is below the static waterline, so the drain is silent. I'll get some more pics and dimensions if your interested. I also fabbed all my parts from epoxy and glass, no plastic here! the thru-hulls are part of the hull and livewell.