Whoa! You gotta mess. Looks like there have been several aftermarket things in that dash...maybe even a CB radio. Based on what I see, there was no factory radio installed (?right?).
Are you ready to paint the dash? Know a good welder? Any patching of the hole prolly won't add sufficient strength to hold yet another sized radio & yet another cut on the dash. Can suggest either short term, or long term fixes. Maybe others have ideas too.
Short term...mount your new unit under the dash. Fab up a cover for the hole by cutting a rectangular piece of sheet metal to over-lap the area. Go to the craft/sewing section of Wal-Mart and get some vinyl to coordinate with your interior. Cover the sheet metal with the vinyl, and velcro in place. You should be able to find some good spray-on craft glue that will secure the vinyl to the back of the sheet metal.
Long term...get to know a good welder, have him weld the holes shut with sheet metal. Then you're gonna have to paint the dash and cut a new hole for your radio.