you have 3 options available to you for your overflows:
1) a hang on overflow such as these at Life Reef
. Which are rated for about 700gph for a single and 1400gph for a double. Either should be fine for your set up. You could get away with not blocking off any visible parts of your tank, but you will see a "box" in the water at the top of the tank. They are typicaly apx 5-6" wide and in the water 3-4" deep. Not a big optical nuisance but it depends on what your going or personaly.
2) a drilled overflow also recommended by FishFreek. This is a hole drilled near the top of your aquarium with a bulk head and plumbing attached and water flows into it as the water level in the tank rises. These are the most "maintenance" free IMO
and also the least visible in the tank. You would not need to block of any of the viewable space of your tank with these.
3) built in overflow. These are the types of overflows seen in "reef ready" tanks but would require one or two ends of your tank be blocked of visibly.
, I would go with option 2.