If you get the 4way loading bearings then it is best to mount the rails on the front face of the gantry and means the z axis mount is simple plate as well.

You only need to mount the rails top and bottom if they are the supported rail type due to the unequal load capacity of the open bearing.

Your triangulation needs to go from one node to another node (connection) or else to will place a bending moment in the middle of the beam. :0)