I had a look at your 3D file and it looks very interesting. The butt joints on the upper carriage rails leaves me wondering how well machined those ends will be but you won't know until you try it, as I did with my approach. I'm not clear on whether the belt is direct drive or motors are mounted at 90 degrees (gear driven). The pulleys look like they are not GT2 type. The precision of GT2 is much finer than other types which I attribute to obtaining an insane level of repeatability with my system. This can greatly impact the ability to use templates to stitch featureless areas. The fewer the teeth the higher the torque I've read. I'm using 20 tpi pulleys with Nema 17 motors rated at 60 oz/in.
If you haven't already, here's a great quick read on the subject of belts/pulleys pertinent to our application:
http://reprap.org/wiki/Choosing_Belts_and_Pulleys
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