My clam has about 12-15mm of clearance, each side, between the 'Clam' and 'Frame' in your sketch. I have simply tightened one bolt until there is 1mm of clearance on one side, this moved the clam 11-14mm to the right in my case. This was all I needed to centre the clam on the windscreen.
Looking at your drawing I would have thought you could do the same?
If the tube welded to the chassis, is the only thing preventing the clam moving sideways, I would consider grinding this in preference to playing with fibreglass.....