Some notes on squareness for anyone who wonders how exact it should be:
On the 500mm machine, the diagonals are over 1m long, so being 1mm off is only 0.1% skew. That’s not a lot, but remember that the millimeter of skew will show up in prints. Where that happens and how much depends on where the skew is with a ratio of Skew*BuildVolumeLength/FrameLength. If the top of the frame is crooked by 1mm, and it is 765mm long with a 500mm print volume, then at maximum 1mm of skew would cause the front edge of your prints to be 0.65mm (1*500/765) farther to the side than the back edge of your print volume.
If the top of the frame is shifted back compared to the bottom by 1mm, with a frame height of 710mm, then the sides of max height prints will not be perfectly vertical and will have a 0.70mm skew across the 500mm print height. The kinematic print bed levels with respect to the extruder, but if the top of the frame is not level with the bottom by 1mm then the tops of your prints are 0.714mm (0.000025deg) out of square.
If you use one of the M5x55mm bolts from the bottom, you can assembly the entire stack from the bottom up easily. Then drive the M5x40mm down from the top when you have the entire stack completed. Remember to wrap the belt around the pulley when you put it on, since it’s difficult to do after.