@PaulS - That's clever how you used the square cutouts in the Y-axis plates as your new cable slots. They were already there because of the previous mounting of the old parts, and just in the right spot for your conversion. 
Did you ever consider keeping the X-axis belt routing the same, with a vertical stepper in its original orientation, but using a double pulley and a belt loop to link the two sides instead of a metal cross-shaft? (One of the two steppers would be removed, and replaced with a double-pulley on bearings, basically acting as a jackshaft...)