Butch, with M-70's and M-700's, their RUM's have "windows" cut in a std. width 3.6" mag box to accommodate the wider case. If Montana chambers RUM's, see/ask how they handle that.
You may want to measure the box width and compare to M-70/M-700's.
Here's a M-70 .300 RUM donor used to build a .404J. The feeding ramp mod was for fatter .404 bullets, the OEM RUM box was not altered.
DF