The Wyatt box is a must have when switching to the RUM case. First, it has "windows" in the sides of the box to fit the larger RUM case and it is longer thanthe original box. I have them in all of my RUM rifles and would not have it any other way.
Whether you use an extended magazine with a 300 RUM mostly depends mostly on whether you want to shoot heavy VLD bullets. As with so many factory cartridges VLD bullets were not considered when the SAAMI specs were set. The SAAMI OAL for the 300 RUM is 3.6 inches and so is the Rem 700 long action factory magazine.