Just in case, I would plan on having two bearing blocks per side on the Y axis. Maybe you could get away with using some of the longer bearings I have seen on ebay. They look like two blocks next to each other. I would also make your Y rails a bit longer than necessary to compensate for the extra bearing.
You would need to have two rails on the X axis as well. That would increase the weight of the x axis. Your accelerations for the y axis would suffer. Also the single bearing rail would let the bearing block rock in a twisting fashion.
Overall, The rails & bearings would be fine for the Y axis but not so good for the X.
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