I should only need to face off about 2mm from the inside of the boss in the lathe to get an easy 10mm narrower arm. Basically the part in line with the 10mm mark will be pushed in to sit where the 0mm mark is and then re welded. I cold go for a bit more but don't want to make the boss too thin. Most people simply hack the arm up so that it clears the boss and re-weld the boss back in where necessary. I want to try and retain the original material in the arm and clearance the boss to suit. Obviously I cannot go as narrow this way but in my case 10-15mm would be perfect, and all I need to get the extra clearance with the rear guards. It also means that I can retain the stock drive shafts as the CV's have much more travel than this.