The CRB has made a terrible decision in it's latest clarification of the FV track width rules. If you do not want to increase the front track width of your FV, I highly suggest voicing your opinion to the CRB.
The maximum track width that is listed in the GCR was there for those choosing to use a 4.5" wheel. It was put in place at a time when the 4" wheels were unavailable. It allowed the use of the wider wheel without exceeding the previous maximum track width. There was no way to legally achieve the maximum track width without using the wider wheel.
The CRB has now regrettably allowed the disc brake kits to be made wider to reach the maximum track width with the standard wheel. They are now also allowing spacers for those who wish to stick with drums.
I believe this is a horrible decision and serves no purpose. Again, I highly suggest those opposed to this clarification submit a letter immediately expressing your opinion.