id think about putting both Y axis rails top and bottom or you are going to lose valuable space...

the way you have them now will have your spindle over hanging further than it needs to buy quite a long way

try to keep your spindle as far back as possible... every little bit will help, it may not alter the flex by much but im pretty sure that each little fraction of stiffness you save will reduce sympathetic resonance or vibration by a sh*t load

good luck