Drools,
I've seen both single and dual trucks per side as well, it depends on what you are doing and how long the axis is. This is on your gantry right. You might want to consider the shorter carriages, I believe you are looking at the extended. On the cncrouterparts site you can down load pdf and dxf files with all the dimensions, you should be able to size the bearings there.
CarveOne, can speak much more knowledgeably about using them as he did a build, actually rebuild if I'm not mistaken, with them. Check out his build thread, it's incredibly detailed and very helpful.
http://www.cnczone.com/forums/showth...t=70901&page=1
Pete