I am not the expert in this matter, but I always thought that the stiffness in those blocks is such that they do not flex that much at all without binding. My Z carriage has four hiwin blocks, and with a 40 lb load, flex was around 0.001". Have you thought about angle iron, tabbed and bolted to the floor, and then steel wheels with a V-notch in the center? This would probably save money over the precision rails. Plus, the mill-scale would protect the anle iron from rust for a good while.
Just a though. Hope you find what you need soon.