As long as the ballnut is directly between (zero distance measured parallel to X-axis ... hard to describe) the two Y-axis rails then I think 130mm will be fine as the aluminium plate in between is very strong and should not flex much. If you have not already try setting the rails/blocks on the Z-axis into the plate - i.e. mill a slot for the rail and pocket for the blocks. That will reduce the distance further though you'll have fun fitting the Z ballnut as that will also require a cutout.