The bolt stop needs to be shortened .110", the bump out in the rear of the mag well cut out, bottom side, needs to be removed and the rear corners in the mag well cut out need to be re-cut.
If the receiver was a standard magnum the feed lips in the receiver need to be opened approx. .015" per side as well.
Pretty much what Roscoe said.....although, some of the magnum actions won't need anything done to the feed lips. Have had a few come through that did and some that didn't need anything done to feed the RUM case.