First of all, double check the pivot to spindle distance of your arm. I've mounted a couple of dozen cartridges of all kinds in Rega arsm over the year without any problems, so its possible that your arm is incorrectly installed. If the arm is mounted in the right place and you still can't get the overhang correct, as a last resort you could lengthen the slots in the RB600's headshell using a small round jeweler's file. You can buy jeweler's files in hardware stores and hobby shops.
