I would consider just using two bushings per side. I think the center one in a 3 bushing per side configuration would not be doing much except perhaps add a tiny bit of stiffness in the middle of your block. If you go with the block design you would need to place the bushings pretty accurately within the block, I can see that possibly being difficult, how were you thinking of doing that?
I believe guide rods used with recirculating ball bushings are usually fairly accurate hardened steel, is that what you've got in there? I also wonder how parallel those rods are...
Hope this helps,