I can see the pdf but there isn't much there to comment on really. Belts are nice and a good idea to drive on both sides as you have. Can't quite work out your rails, are they bearings running on steel flat?
Good bipolar drivers here:
http://hans-w.com/cnc.htm
Or buy a Xylotex unit.
You might want to be a little more specific to get a response.
Graham