http://www.mycncuk.com/attachment.ph...6&d=1451863790
The one where you can get the bearings the furthest distance apart.
So this one is.
However, in that photo the bearings are closer together than they need to be. You want two bearings as low as they can go and two as high as you can get them. If the top of the plate the Z axis bearings were attached to was extended upwards you could even get the top two higher.
Plus with this design if you are cutting things close to the bottom of your gantry, you don't have the long moment to your first two bearings. The other way, there is always a long moment to your first two bearings. Obviously the further apart the bearings the less movement you will have OR the longer your Z axis assembly will be. So the rails need to be longer this way with the bearings spaced further apart.
440mm is alot of travel. That's over 17 Inches. So the gantry and every other part will need to be very sturdy. Are you thinking for a 5 Axis machine?
Also, for this kind of distance, flat plate for the Z isn't good. Flat plate is easy to bend / deflect. Square box tube or at least stiffeners attached at 90 degrees to the plate are required.
Are you planning on a pneumatic counterbalance?
So this video doesn't belong to me, it's not my machine, I'm just posting a link. You can see that he has mounted his Z axis rails like this. Probably one of the best successful high performing DIY CNC machines made IMO.