High gantry sides on less than rigid rails can lead to a series of short jumpy steps - porpoising is the best way I can describe it. I have seen machines made from MDF and unsupported pipe that showed this and you could hear the spindle loaded and unloaded as the wave action worked.
Can you design your X axis top rails to be higher - ie closer to the Y axis gantry beam to reduce the overhang. By doing this you can keep your high Z axis clearance without the associaited problems. This design element can incorporate a wall (shield) to keep the sawdust, etc from speading over your workshop and X axis rails.
I'm not sure about 12 inch gantry sides as the trade off is a loss of X axis travel but 200mm is about what I work on and 8 inches in your currency.
Another point in design is to get the centre of the spindle as close as you can to the centre of the X axis bearings. In your drawing add the spindle and mount to the Z axis plate and work out how your design looks for centering over the bearings.
Centering the spindle to the X bearings balances the gantry directing the forces through the bearings rather than a lever pulling or pushing on one side of the bearings.
The second advantage is it brings the cutting envelope into the centre of the table. In extreme examples I have seen designs where the back 12 inches of the table is never used and the spindle projects over the front of the table at the other end. A cranked gantry side is the easiest way to overcome this so if you have access to machines that can cut plate at work then you might want to consider this. If not then jig your Y beam and Z axis components to get it the best you can.
Last think about mounting jobs on your table. It is rarely considered at machine design time but ends up being the biggest pain when using your machine. Vacuum, T slots and cross table clamping are a couple of ideas and I am sure there are more. A lot just wack a spoil board on the table and screw the job into the spoil board but this is not the answer for everything - eg engraving the lid of a jewelery box.
A couple of ideas for you to think about.
cheers,
Rod
Perth, Western Australia