Contact the RCBS customer service - IMHO some of the best people in the business. Ask about product # 99200 (I think) which is the shellholder adapter that you need. I makes the case stand a little further away/taller than an "original" holder, but it works well. I have an old Lachmiller press that I used along with the adapter to load my 300 WM, and it worked very well.
All in all, total price including shipping was under 25 bucks. A lot less than a new press, so it gave me more money to spend elsewhere.....