Bob you would use the wedges between the housing and springs if you needed to change pinion angle. In this case with the slapper bars they are used to help bring the front of the slapper bar closers to the spring.
If your going to cut the slapper bar itself, I would recommend finding some rectangular tubing the same size and extending them so the rubber snubbed hits right under the spring eye.
Remember to put some weight in the car to match the drivers weight to be more accurate in the whole shimming or cutting.