It has been said many times before, but if you are planning on using epoxy to level the x axis beams, you are going to want them wider than 50mm, 80mm works well. Also I wasted a lot of time with bolts - just weld it all, would be my advise. Another lesson from my build is get box section with thick wall - 5mm not 3mm as it will tap much more securely. Why do you need so much travel on z? it will be hard to maintain rigidity at that length. I also have real issues accessing the y and z axis ballnuts to grease them - make sure you figure that out now! Good luck with the build, this forum is great for keeping you on the straight and narrow!