I'm using much the same tactics as your delrin bearing design. I've found that by using hand tools that it is very difficult to get the bearing blocks perfectly aligned. However, make sure you are precise as possible and you should be fine. Having said that, I don't see any means of adjustment for you y-axis rails.
My other suggestion is to carefully consider your horizontally aligned rails of your y-axis. In that configuration any weight is unevenly distributed on the the leading rail. IMHO it would seem that deflection of that rail will be greater because of the torque. It the rails were vertically aligned the torque should be lessened because the zaxis would be closer to the rails.
Here is a link to my machine in progress... your design is similar yet different.
http://cnczone.com/forums/showthread.php?t=4716
Cheers,