The stop Steve and Damon are talking about shows up as "A" in the photo below. When shifting into 3rd and 4th, this stop bumps into and rides along the downturned metal lip of the lever support mount "B."
When shifting into reverse, you push down on the lever and the stop slides under the metal lip, allowing the lever to move further to the right (and the gear shift control rod to rotate further).
From what Damon and Steve are saying, the front-most portion of this lip needs to be cut away to allow an easy shift from 4th to 5th gear.