Roger Ram Rod has it right. His just needs to be fastened to the dash frame.
Here's a couple pictures that may be helpful.
You can track down a bracket or make something that does the same job.
There are at least three brackets for sale on Ebay now, or post a wanted ad on the forum and you might get one for a bit less.
They really help to support the back of the radio. Fastening the front of the radio through the dash openings isn't really enough to hold the radio securely.