I have made a few modifications to the bearings on my CNC Fusion X3 kit. I didn't like that there was nothing for the bearings to load against except the remaing part of the threads where the shafts were turned down. I put another split collar on the backside against the threads. There was plenty of length on the X axis end but the Y axis bearing block had to have a couple of pockets milled out to inset the collars to have enough shaft left over to attach the motor coupler. I also had to replace the thrust bearings with slim roller thrust bearings. The bearings don't have to be replaced on the X axis to fit but I replaced them with roller bearings anyway.
Additional posts with pictures follow.