With the Hiwin's, it doesn't really matter that much.Is there an specific place where the spindle is mounted in order to optimise the distribution of forces. Should the spindle be in the middle of the Y rail?
You might want to make the side plates a little wider, to increase the bearing spacing a bit more.
The thicker the plates, the better. Maybe 15-20mm?
And I'd get a 2.2Kw-3Kw spindle. The 800w is too small imo.