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.
View attachment 1716223949
View attachment 1716223951
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.
1968-69 Barracuda Bracket RADIO MOUNT TO DASH 2864790, 1970-1971 Duster Dart | eBay
1968-69 Dart Cuda Valiant A-Body Radio Rear Support Bracket 2864790 | eBay
1968-69 DART CUDA VALIANT A-BODY RADIO REAR SUPPORT BRACKET TO DASH W- HARDWARE | eBay