For bulb arrangement, I would not put 1 of only 2 actinic bulbs in the back. To maximize the visual color appearance of your corals, you want the actinic bulbs near the middle / front of the tank, where the actinic light will be closer to your viewing point.
There's a much wider variety of T5 HO bulbs (54w) than just 10,000K and actinic. I'd seriously recommend reading the T5 Q&A thread on RC -- lots of bulb combination listings there. Here's the link to the most recent portion of it, as it is a very long thread and has been split many times (links are provided at the beginning / end to get to the other parts):
http://reefcentral.com/forums/showthread.php?s=&threadid=1169341
If you go with retrofit kits, be aware that some individual T5 reflectors are slightly wider than others. That said, you may not be able to be fit 6 T5s in a canopy for a 75g (depending on the canopy's design), especially if you decide to use IceCap SLRs, which are in the upper range for width.
There's a much wider variety of T5 HO bulbs (54w) than just 10,000K and actinic. I'd seriously recommend reading the T5 Q&A thread on RC -- lots of bulb combination listings there. Here's the link to the most recent portion of it, as it is a very long thread and has been split many times (links are provided at the beginning / end to get to the other parts):
http://reefcentral.com/forums/showthread.php?s=&threadid=1169341
If you go with retrofit kits, be aware that some individual T5 reflectors are slightly wider than others. That said, you may not be able to be fit 6 T5s in a canopy for a 75g (depending on the canopy's design), especially if you decide to use IceCap SLRs, which are in the upper range for width.