Which way should I go?

A little bit of plumbing involved but works well.

http://www.youtube.com/watch?v=65yVr7DiDls

Id drill a couple holes in or around the upper back wall corners, one through the top piece for a return and call it a day. Most all my drilled tanks are like this as i hate built in overflowboxes. I use normal bulkhead screens on a couple of mine as well as pvc elbows with homemade screens on others. Just need to consider water height before you drill, elbows are good for adjusting that but look worse than the screen alone.

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Just a couple examples of what i have goin...nothin fancy but they work.

Single drain on a 300

Double on a 230

Autodrip overflow on a 55

197 right side/ left side just the same.
After closer inspection the dual drain 197 runs about 1/2" from the top of the bulkhead making the waterlevel 3/4"-1" from the top of the tank with 1800gph and 1.5" drains. Ur probably lookin close to that for urs id guess.

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This tank is acrylic you could put the overflow slots into the back wall or side wall. Build the box on the outer surface of the tank. This would be the cleanest setup nothing inside the tank

I vote coast-to-coast external overflow. Cut down the back wall and build a box along the back outside.

What about putting a pump in the tank to flow into the sump? A lot of stingray breeders do that.

good luck!!