Okay,

Further drawing, as promised. With additional information.

Uprights of gantry are currently looking to be either 20 to 25mm (or 3/4 to1") thick. The rails are bolted down to 25 x 38mm bars that are going to be bolted to the back plate (12mm). The Z base plate is 20 (or 3/4") . I hope the way I had the jpeg done allows folks to see more of what is going on without getting a headache. This is a rough sketch done with ViaCAD 2D/3D and no I don't own any part of the company nor have vested interest in it.
Click image for larger version. 

Name:	Y axis idea N2.jpg 
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Any and all opinions are please.

Michael