On my Isuzu Rodeo, I had the installer cut the screw-in bracket. This was mainly to ensure the Empeg sleeve wouldn't rattle against the bracket. The bracket was cut in half, so only the pocket is held in by screws. See some pics of my install here. The Empeg is held in with the sleeve. It fit perfectly.

EDIT: I just noticed the last part of your post. To stop the sleeve from sliding out, you bend the tabs to make it grip your dash trim.