The top rollers are similar to the bottom rollers, but a bit smaller.
The type is SG15 U Groove 5x17x8mm Sealed Ball Track Guide Bearing.
Ebay link:
-
http://www.ebay.com/itm/400561381584
The top rollers use an M5 40mm bolt and two M5 4mm spacers on each side to get the right width and position (I've not received the spacers yet, so I just used some washers in these pictures).
The mount is a bit different than the original CAT 390 (and most other excavators). Normally, the top rollers are only mounted on the inside. I guess this is to avoid rubble being stuck under the roller as it slides down on the tilted suface of the rail. But for this build, keeping it simple, I've just mounted it on both sides like the bottom rollers.