Y and Z axis set up.....calculations

[dudz]

My existing machine works well milling aluminium, but the Y axis has 25mm unsupported rails, and the Z axis is some obscure setup which needs changing for the better to make the machine more accurate/stronger.

Here is a photo of my machine as it is now....

The frame is 45mm wide extrusion.

I have been through this with Jonathon and he has helped me a great deal , but I am confused about a few things.
I need a Z travel of 150mm.
I plan to put a 15mm thick alu plate across the entire width of the Y axis and mount 20mm supported rails on top to give me a Y travel of approx 320mm. I want to use new 1605 ballscrews for both axis. Jonathon kindly came up with most of the parts that I require ;

1set 1605-L330mm ballscrew with nut and end machining ( Dimension F needs to be machined to 25mm)
1set 1605-L550mm ballscrew with nut and end machining ( Dimension F needs to be machined to 25mm)

2set SBR20-L320mm rail support
2set SBR20-L515mm rail support
8pcs SBR20UU



Plus the aluminium for the Z axis front and back :

320 x 160 x 20mm x1
180 x 160 x 20mm x1

I done a drawing for what I think it will look like, but I cant work out if I have enough Z travel, plus the best way to mount the Z motor ? as there doesn't appear to be enough room for it on the front or back of the Z plates. I might have calculated something wrong or there might be better ways be moving components around.

Diagram : (the longer Z plate being in front of the shorter Z plate)




I estimated the Y plate to be 515 x 240 x 15mm

I can grasp the Y travel, thats easy enough.
Any help would be great. Its confusing the hell out of me !