What's the CFM of the 6" fan with the 600w kit?
If its around 260 or above I'd go for that but you may want to introduce a booster fan as an intake so your tent doesn't try to unfold.
IMO a 400w would need a decent 5" in that tent.
I use a 400w in my closet with 300CFM fan on a speed controller to control temps.
I used to use a 440CFM fan for my air cooled 600w lighting rig.
My closet is 7x3.2x8 and I use 3x3 area for my 400w but I did run 600w over the same footprint.
J