I had a bunch of thoughts, excuse the rambling in the following reply...
I'd recommend doing half regular blue and half royal blue. That's what the AI fixtures use. The royal blue color by itself is hard on your eyes.
56 might be more than you need, but without a PAR meter, it is tough to know. I'll end up with about that equivalent when I add blues to my fixture. I'll have 126 LEDs over a 125 gallon. Use dimmable drivers and you can dial back the current. Running at a lower current will make the LEDs last longer.
A large heatsink will let you spread the LEDs out a bit more and give more even coverage. That will be a bonus if you are using optics. You will also want to cluster the LEDs in groups of white/blue/royal so that you don't get so many shadows of different colors. The LED stars are only about an inch across, so you can put them pretty tight to one another. I got a 4.25" wide, 72.5 inch long heatsink from HeatsinksUSA, which fits perfectly over my 125 tank without needing any extra bars to brace it. For a 60 cube, you would want a much wider one, or maybe several bolted together in roughly a square. With LEDs there is a trade off between even light and number of LEDs. There are getting to be some really high power LEDs out there (10+ watts), but you can end up with spotlighting. I think it is better to go with more lower watt LEDs (CREE 3 watts), unless you have a deep tank or are planning to mount them up high. This becomes even more of an issue when you have to use three different colors mixed to get the "right" color.
Depending on how high you plan on hanging the fixture, make sure the rows of LEDs are not too far apart, or the optics might result in shadows. Rows 3-4" apart work fine with my fixture only 5" off the water, but I think a 6-8" spread would cause some "spotlighting" with the fixture that close. I have a 6" gap between the groups of LEDs over top of the two braces in my tank, and the shadow lines are visible. If I had to do it again, I would replace the black braces with clear acrylic and spread the LEDs out evenly. If you mounted yours higher, you could have the emitters farther apart. Since you have a cube, you could also use wider optics in the center of the tank and narrower optics around the sides, which would give you better mixing and spread of the light, while still not losing so much out the sides of the tank.
I used adhesive rather than drilling.
On a 24" cube, I'd consider a roughly 16"x16" fixture with 16 groups of three emitters in a 4x4 pattern roughly 4-5" apart. That would give you a total 48 emitters, with roughly 120 watts of LED power, all of which with optics could be directed into you tank. I'd consider none or 80 degree optics in the center, and 60 or 40 degree around the sides, depending on how high you plan to hang it.
If you really wanted to take full advantage of the optics, you could use separate heatsinks and tilt the outer ones so that they direct the light inwards, which would lead to even less light spillage out of the glass.
