I found a pair of 16mm x 450mm ballscrews on ebay for $130. These were machined with 8mm motor connectors and 12mm bearing steps, and came with 8mmx8mm motor connectors, and 12mm bearing endblocks, pretty much all the hadware needed. They took a couple of weeks to get from china, but the price is good.
Attached are some pictures of my conversion. This turned out to be very simple after a few false starts. I designed bearing holders in freecad that are just 28.05 and 26.05 cylinders cut out of 30mm and 28 mm cylinders. The inner cylinders are at x=0,y=0,z=2, and the outer ones at 0,0,0, A third cylinder 22mm in diameter, at 0,0,0, cuts a hole in the center, leaving very nice bearing holders. The bearing snap into these and the holders fit into the existing holes in the Z top and bottom brackets. Pretty cool, huh? Peircet probably thought of it first:)
The exisiting Z nut is replaced by the ballscrews, and a printed adapter sits on top of it to attach the metal X axis bracket. I did need to trim the bracket and the bearing holder for the support rod for clearance, but no major surgery was needed. I think I could even get rid of the support rod the ballscrew shafts are so sturdy.
The only tricky part was how to attach the motors 5mm shafts to the 8mm coupler. I just happened to have some aluminum tubing with about 1mm walls, so I cut a short section, cut a slot down one side, and pressed it over the motor shaft. That took care of the motor coupler.
All that was left was firmware changes to handle the different pitch on the new screws. The stock leadscrews have a 2mm pitch, these are 5mm, so 1600 x 2/5 = 640. You can change that in the Configuration menu, or in Configuration.h in the firmware. Be siure to change the Z axis and not the extruder settings, or weird things happen.