My understanding is: the spindle is tilted in y direction, so you would not get a plain surface when face milling, right?
In this case: You won't like the answer, but:
I have had the same issue with a similar machine (Not:Tormach). There is basically no reasonable way to fix this and keep the rigidity of the machine in place. I know some persons fix it by inserting a self-made wedge between base and head, but it causes a loss of rigidity. I returned my machine.