I think the printed parts need to be printed according to the belt that you are going to use. There is a option in the config file where you can change the tooth distance and then generate .stl files for the x carriag and y belt holder. This would print parts compatible with your belts. The pulleys need to be compatible with the belts as such. Use metal pulleys for best results.

So it goes like this printed parts (x carriage, y belt holder )-----compatible with---------> Belts (tooth distance)----compatible with------>pulleys (preferably metal).

@kingoddball - did you print i3 parts yourself or got them from somewhere. I have printed i3 parts from prusa vanilla files as they were. I haven't got the belts in my hand till now, but i have ordered gt2 belts. I hope they fit into those parts.