Bah,you can use the 600 watter with no problems,like mentioned keep it 3 ft then check heat with a mercury thermomter at the top of the seedlings,be sure the temp is around 75 degrees & all will be well.
Whoever said clones/seedlings will stretch under a 600 watt hid is tripping out badly,a room full of frigen cfl's wont put out the lumens of a 600 watt hps no matter the height,i ran my last bunch of clones under a 1,000 watt hps with excellent results simply because i was too lazy to take them into the mother room,hid's work great for clones & seedlings.