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(http://i.imgur.com/ksoLDEy.png)
I gotta second Oobly�s recommendation to center the spacebars under the edge between G and H. It�s really silly to have tiny split spacebar not centered (makes one of them, in this case the left one, basically useless).
While you�re at it, I�d recommend making each one somewhere between 1.75 and 2.5u wide. Split 1.5u is just a bit too small to be comfortable to type on for many hand shapes. (Or alternately keep the 1.5u spacebars but add an extra 1u or 1.5u key between the two, and use it for some relatively uncommon function.)
You might want to think about which fingers you�re expecting people to use to press the other bottom row keys. If you want 3 keys on each side to be pressed with the thumb, you might as well make those Alt keys 1u wide, so the further keys are easier to press. Ctrl in the corners and caps lock wasting valuable space is basically the worst thing ever, but IBM turned it into a law apparently, so do what you have to do.
One last bit of advice: either make the keyboard Alps + MX compatible, or make it a flexible layout that supports multiple switch arrangements, but don�t try to do both.
I must sit in a weird position or something, because when I press the spacebar, it's always directly below the 'N' key. My left thumb, which I never use to hit the spacebar, would hit it directly below the 'G' key. Although my index fingers rest on the 'F' and 'J' keys, my hands seem to be positioned in the "geographical" center of the 60% keyboard, not in the "hand position" center.