I don't think its too much just not to close to the water do to overhearing issue, I would use a 20K bulb. Do you have a chiller? if not and if you end up needing one I have a coralife chiller for tanks up to a 75gal.
Years ago I had an DE (( Current SunPod if memory serves )) 250 watts over my 33 cube, and was very pleased with it. That fixture had small legs, and still didn't have any issues with heat.
Can see the distance b/t the water and lights. (( did have a fan over the sump that turned on when the temps got too high ))
I started with the 400 watt Radium and have to say if you can afford the heat, electrical and bulb replacement its one of the best choices.
Sanjay Joshi did a spectral analysis of MH Bulbs back in the day. I used this information to replicate the 400 watt Radium while only consuming 118 watts with 10% more PAR Using Cree XT-E and XP-E2 LEDs.
The huge advantage LEDs have over MHs and some T5s is they can be controlled to provide a natural sunrise, sunset and moonphase.
Here is a pic of our P47 fixture. The Retro version is a lot less expensive. I attached a pic of our latest fixture (release December) and given the intensity, controlability and efficiency it will give most die hard MH users a reason to switch.
If you have any questions about LEDs feel free to contact me