Before you start shimming on the carriage, determine whether or not the lead screw is parallel to the side of the bed. As the machined side of the front way is well above the screw, this may be difficult to measure directly. So position the carriage near the headstock and near the right end of the bed. In each position (motor not running), slowly close the halfnuts until the inner nut just touches the leadscrew (rock the carriage back and forth as needed in order to be able to engage). Make a pencil mark on the apron to indicate the angle of the lever at touch up at each end of the bed. If the tailstock end or the screw is closer to the bed, shim the support bearing to make the arm positions equal. If the reverse is true, I don't think that you can move either the QCGB or the left support bearing in the horizontal plain. So unless a PO had previously shimmed the right bearing carrier giving you something to remove, the only solution would be to machine the mounting face of whichever type bearing carrier you have.
Once the screw is parallelto the bed, proceed with shimming the scroll and nut carrier away from the apron. If this results in the lever hub binding against the face of the apron, you will need to also shim in the square hole in the hub.
