That's not what needs opening out...
Eyeball the third picture at this link
viewtopic.php?f=9&t=13305&start=60#p181250You have the tube things that nobody can figure out the reason for on the end of the chassis rails, and then the mounts for the body & bumper just outside this.
The winchbumper has locating tubes which go inside the oens on the chassis, then plates which fix to the bumper mounts and bullbar mounts. As it stands things seem to line up just right on the drivers side, but on the passenger side it looks like the locating tube is too close to the plate for the bumper mounts, and it won't slide all the way home.