As you've drawn it in 2. if the gantry moves to the right it knocks the red block over...oops. You would need the spindle further down, but then the Z-plate is longer adding to the flex. The better way to do it is to mount the rails to the plate the spindle fixes to, and the blocks to the gantry. Rails move up and down, blocks fixed. You'll find it mentioned in more detail in quite a few places on this forum.

You're in the same situation as me with regards to only occasionally needing more than 30mm of Z travel. Remember to add a bit for tool changing if you have not already.
Since my Z-axis has 400mm travel continuously operating it at maximum travel to do thin parts is just asking for trouble due to the large moment it applies to the gantry. I countered this by eliminating the sides of the gantry so that the Z-axis works below the level of the X-axis rails, and having the height of the bed adjustable so I can optimise it for different height parts. See it here:

http://www.mycncuk.com/forums/showth...outer-building...

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