I didn't see it mentioned, but you will also need to cut a hole on the outside of the frame rails on each side for clearance of the nut on the front spring hanger. After installation, those nuts will actually be inside the outside flange of the frame rails.
When I installed the 3/4" MP kit on my '71 Duster, I cut the original spring purches from my housing and reused them. Without tubbing, I was able to fit a 305 50 15R, or my 28.5 10.5 15 slicks, on Draglites with 6.5" BS easily without rubbing. That was with a stock width B-body Dana.