I'm not sure if early GCIII used monarch superspeeds like the GCII or not. If they did the subrails will need to be modified to use K55 cushions. Their is no direct replacement for the old Monarch Superspeeds.
Not a huge fan of having cushions that only fit that table. Proprietary parts like that can cause issues down the road. It may not be an issue but companies are more apt to abandon making products that only fit one table if there isn't enough call for it.