That looks stronger than previously :)

Put the rails on the Z-axis and bearing blocks on the Y axis box as that's stronger. With how you've drawn it the Z plate will bend wherever the Z-axis is. Third time I've posted that this week...wait you had it like that in post #21, why the change?

The centre of mass is not ideal, but it depends how you look at it. Ideally you would have the contact force at each bearing equal. If you calculate the forces and compare to the ratings of the bearings (http://www.mycncuk.com/forums/showth...near-bearings-... I should find the spreadsheet I did for that) then you will find its not significant. If you're drilling something the forces will change and actually get more balanced.

You could compensate by mounting the gantry further back between the bearings so that the spindle is closer to them. Ideally work out the centre of mass of the gantry assembly, minus X bearings, and mount it such that that point is directly between the two bearings. That's what I did, but couldn't get it that far back so ended up about 30mm off the centre of mass.