I used an alternate frame-vertex for these parts found on thingiverse.
Attach two frame vertices and the z idler bracket to a stud. Don't worry about getting the nuts that hold the idler bracket tight yet. They will be adjusted later. Measure the distance between the frame vertices and make sure it is equal to J1 (200 mm).
Do the same thing with the z motor bracket.
Add another frame vertex and two more threaded rods to make two equilateral triangles. Once again make sure the distance between the frame vertices are equal to J1 (200 mm).
Use the two top threaded rods to connect the two triangles. The distance between the frame vertices along these rods should be J2 (140 mm).
Attach the y idler bracket with two threaded rods. Make sure to put on the nuts and washers to hold the y-bar-clamps later on. I used an alternative y-bar-clamp which also has to be installed while building the frame.
I found it helped to put the idler on first and then tighten the two nuts on it. Then I have a nice lever to turn the rod with.
Make sure the distance between the frame vertices is J2 (140 mm).
Attach the two threaded rods as was done on the y idler side. Don't forget the washers and nuts to hold the y-bar-clamps.
Install the two threaded rods that support the z motor and idler brackets.
Use a string to line up the z-axis. See the Mendel instructions for this technique.