Unless the centerbrace is REALLY REALLY perforated you will see a serious shadow. I had my 75G with the AGA brace for 3-4 yrs as a reef running 1 x 400 SE + 2x 96W PC actinic. I got used to it, and arranged the rockwork to de-emphasise the shadow in the center of the tank, but overall, honestly, any centerbrace will take away from the one lamp set up.
Tough call, I don't know what to suggest. If It would help, I might have the parts on hand to loan you a 400 SE 10K retro to try out.
Edit: I forgot to answer, About 12" above the water on the 400's, but thats a rough estimate since I haven't run one in a year and a half.