Oh ... I wouldn't recommend cutting it because I think it would ruin the cab and probably look pretty funny. You actually may be able to mount the monitor tilted, rather than straight up and down which would require you to remove the shelf. But I'm not sure if the cab is deep enough to accommodate that either (as the monitor may stick out past the rear door). You would have to take some measurements.
If possible I would just look for a cab that has a vertical orientation. These types of mods don't usually work out very well and usually end up ruining the cab.