a.) On a 24" depth yer gonna want 250w halides... 2 - 250w halides would suffice, because fitting 3 in 48" is gonna be tight...
b.) That ballast MIGHT be a bit too hot to go right beside the tank... The PFO I usta use was hot enough to burn you if you touched it, so vent that enclocure very well..
c.) That long top is gonna be a pain to work with... Break it up into segments just big enough to hold each reflector/socket... If you make it like say, 25", you can add a couple 24" dual bulb NO fixtures for the actinics...
Nice layout otherwise =) I might honor you with a 3d rendering of it this weekend, if I get inspired enough... Below is one I did of the hood I built last summer....