1st Build, Second machine

[JAZZCNC]

Put the rails on the front plate and the bearings on the back plate. This way the rails help stiffen the front plate.
The carriages then go low on the backplate and it makes for a very stiff z axis with max support where it's needed.! close as possible to the cutting end.!

Edit: Also if you pocket the bearings into the back plate slightly you regain some of the lost stand off from gantry. Also you can use one of the pocket edges as a datum ref point for alignment of the bearings. If you also do the same with the rails into the rear of the front plate, (but not quit as deep) this as the same affect and also another datum ref edge.
The net affect of both brings the stand off from the Y axis down slightly and also creates accurate ref edges to work from which are essential for setting up profiled linear rails correctly.

Hope this makes sense.! If not just ask and i'll try to explain better.