Be a little careful because they changed the radio's from 1977 to 1978. The Delco AM/FM cassette's that you'll see on ebay are most likely from a 1978 or after car. The difference is the speakers are not grounded. If you get a 1977 radio, then you won't have this problem. But you need to make sure you're getting a 1977 radio, or you'll have to change the speakers to the newer non grounded type. Otherwise, you'll blow the radio.
You also need the radio bracket that holds the radio to the dash frame. If yours is missing you'll have an awfully hard time finding an original bracket because they usually get tossed. You'll have to make one.