Hi Robert !
Referring your 3D design...
1) Such a long Z axis move and just one ( ok very massive) rail.... - I think it would be better taking two smaller rails/ bars with the max. distance possible!!!)
2) the sides of the y Axis seem very thin to me - in relation of the length ..
possibly this causes an elastic ( parallelogram spring) area that may decrease the precision of you Y axis under load!
3) The X axis fixation to the sides / sliders /plates may be overdefined!
The Y bar has a certain length ( + tolerances) the front- and rearbar of the frame as equal. for you use those metalsheets to fix your sidebars to the front 6 rearbar this construction is not able to eliminate the tolerances without deforming the sideplates...
you wont get exactly the same distance between
(2xsidebars+ 2xThicknessofsideplates+ 2xThicknesofrails+ LengthofYbar+2xThicknessofsliders) = (lengthoffront-/rearbar)in case of tolerances... if you solve the fixation - fron to sidebar in another way so you are able to vary the distance of the sidebars - you can eliminate the difference in distance....
Please take this for constructive critisism - just want to help - so think about it and leave it if you decide to - just an idea!!!