Ross what exactly do you mean by make the Y axis wider? doesn't this mean that the distance between the cutting tool and the back of the Y axis will be further?
If you look at the last image of post #8 then you have dimensioned the beam as 126mm deep and & 70mm wide (guessing back plate is 20mm) the 70 mm is the bit increase. yes this will increase the distance from cutting tool to back of the y axis but you have gained a bit of space by rotating the rails to the top and bottom and you could allow the very back to over hang the rear gantry bearing a bit as the resultant twisting force from the z axis on the yaxis will act in the centre of the beam. as long as the rear X axis bearing on the gantry sides is behind the centre it will be ok.

If you are going to belt drive the z axis motor then it might also be worth mounting on the back of the z carriage plate so it is lower on the machine, whilst it is not going up and down with the Z axis it is still being accelerated back and forth on the Y axis, bringing all the mass closer to the centre can only be a good thing :o)