I am 90% convinced the rear rods are three eighths of a twelfth of the King's foot (9.53mm, if you use the customarily accepted King).
Replacing the bushings with ones from Solidoodle should be as easy as pushing out the old ones (may require a bit of force - judicious use of a hammer and a piece of wood with a hole in it is pretty good), and then pushing the new ones in (maybe be a bit gentler in this direction, tap with a wooden block or a nylon fitters mallet!).
I have not come across any common methods of mounting say linear bearings on the bed for the Z axis, so unfortunately you may have to design the application yourself (would be awesome if you'd share it and your experience with us too, if you do so). Given the current rods are close to 10mm, it would be easy enough to drill the holes in the frame out to 10mm and use 10mm metric linear bearings and rails if one preferred.
SD3. Mk2b + glass, heated enclosure, GT2 belts, direct drive y shaft, linear bearings, bowden-feed E3D v5 w/ 0.9° stepper
Smoothieboard via Octoprint on RPi