The 3 rails that make up the RailCore X/Y stage are minimalistic, but still have their challenges. The challenges here are as follows

• The dual X rails are mounted directly on the frame extrusions. Getting them to be both parallel and planer can be a challenge. A square frame is critical, but there is slop in the screws that hold the rails to the extrusions.
• The Y rail is simply constrained at the 2 ends by the X rail carriage blocks. The belts help to maintain the rails perpendicular to each other, so it doesn't take much here. 

I've considered putting the X rails on top of the Halo. The challenge is all the belt pulleys would have to lift off the halo to clear everything. Not a big deal, except the stepper motor won't really work being mounted on the halo, as the motor shaft is too short and you don't really want to put that much of a cantilever'd load on the motor shaft. 

Railcore gets away with a single block on each X rail. They do use the long block. 

One idea I'm considering is using 2 short blocks on each X rail. This would add about $40 to the total build cost (Misumi high-temp SSE2BST13-395).

Alas, no decisions yet. At this point, I have things I want to consider, but will be moving on to the Y Carriage. 

Lots of decisions to make here, which extruder, hot end, height sensor, etc.