Ok, cool. I'm going to go with the double then- thanks for the input. I'll have to order from another vendor- I hope they are comparable. The "TEN-HIGH" units worked out really well. The new vendor is off ebay. 20mm is a bit larger than the lead screw diameter that came with the machine. The Z was the smallest at 16mm, but the Y is also a 20. There would be clearance issues elsewhere if I went larger, and 20mm seems conveniently standardized, so I'll go with it. I'm not super worried about loading, probably just being a little OCD. In a perfect world, each screw would be precisely proportional to its load, but I think this should work. Still could upgrade the bearing spec in that block, but we'll at least give these a try.