The mag-drives are nice pumps, I have 3 or 4 of them going right now I think, of various sizes. The little airline on top of the overflow box is for priming it. You can either hook it up to the venturi hookup on a powerhead (ok) or like I mentioned before, hook it up to an Aqualifter (much better).
That being said, I would tend to go much bigger on the pump and overflows. Difference between one drain and two is just flow rate. One 1" bulkhead will flow about 600gph peak. So, an overflow with two 1" bulkheads will flow around 1200gph. Like Ace said, I'd be more inclined to use something along the lines of a Mag 12, with two overflow boxes. On a 6' tank you'll get much better flow with a drain on each end. I've never used the SCWD devices but they look cool and I'm thinking of incorporating one when I do a closed loop on my 55. But, with a 55 for a sump, you shouldn't have too much trouble handing 1000-1200gph there.