[mps9506]

You best bet will be to drill a hole in the side of the cube and build a durso pipe on the side. The reason you don't want to just drill a hole anywhere in the side is because it will drain all the water out of the tank, and if you just use a bulkhead on the side you will have noise issues and it starts sucking air and water down the drain (think toliet).
As far as flow, it depends on the size of your drain line. Assuming you use a 3/4" or 1" drain I would say 200-300 gph is all you would want for that size tank. The flow to the sump will only be as fast as your return pump pumps water back into the tank.