Thanks for the replies, everyone. I still think it might be more cost effective to use through hole LEDs, but not as much as I thought. Anyway, it wouldn't be that much more expensive to use higher power LEDs.
I hadn't really given a lot of thought to heat problems with the through hole LEDs. Do you think it would be OK if I mounted them in holes drilled in a piece of aluminum? I could always add a couple of computer fans too if needed. That would take a long time though, to drill 144 holes!
I do have one more question (for now!). Do you guys think I could use all 6500k LEDs or should I add other colors? I know there have been other threads on this, but everyone seems to have a different answer! Does adding other colors like blue, red, etc. make the plants grow better, or does it just make the tank look better? And what would be a good ratio for combining LEDs of different colors?