Not bad for a first build, well done
for the 8pin, try dremelling out a suitable sized hole in the mobo tray, the cable goes out the rear with the rest the up to the back of the mobo where you plug it in, nice and neat like,
**Edit, to give you an idea of what I meant, the 8pin on my rig, the rest is hidden by my mobo backplate but it goes to the Psu as I mentioned before
Another note though, do you have a front fan blowing onto those hard drives? If not, I'd put one in if possible. If they are all 7200RPM drives, they can get hot if you have a group of them all together like that (especially if you RAID them). Hard drives can be temperamental as it is so don't let heat become another problem if you can help it.