I just went through this with my 55 Gal. set up....
It did take a while for the sand to settle, 3 weeks to be specific, but when the water did clear, it there was a nice white layer of sand at the bottom and my moving my live rock around and rearranging everything didn't cause a huge cloudy sand storm in the tank. Once the bacteria takes hold of all the dust and settled the sand, you've got a pretty nice setup.
My tip: put your heater, pumps, powerheads, skimmer, etc. in place. spread hyour southdown sand into the tank the way you would like to see it, e.g. peaks, valleys, etc., then fill the tank. You'll get a nice giant tank of soup... BUT remember, patience is rewarded.
Heat up your water, add your live rock.... In a week, my tank started to clear... I used a kept the settling sand off the live rock daily and after the 2nd week, ran my power filter to get rid of the top film layer and any dust. I turned the skimmer on after the end of the 2nd week and by the third week, the water was crystal clear.
hope that helps...