I used M3 T-nuts, solid not the drop ins. And used ALL the holes in the rail.
Yes, it was a PITA. Had to suspend the extrusion on 123 blocks on a granite surface plate, and indicate repeatidly to keep them within a .0005" of each other on the gantry while tightening every SHCS. The Y and Z rails were easier as the rails only needed to be parallel to the face of the 2060.
Thanks, next time I wont buy MGN chinese knock offs, buy rails with 2 trucks (blocks) if you need two. NEVER try to add them like I did. Had to repack the bearings and throw away a half dozen blocks that were FUBAR.