In '68 (remember Malibus and SS's all had the same radios) you could have ordered:
AM pushbutton radio
AM/FM pushbutton radio
AM/FM pushbutton radio with multiplex adapter (this mounted below the dash to allow the am/fm radio to receive FM stereo broadcasts)
Tell me what kind of antenna "remains" are in the car if any, from that I will be able to give you a better idea what radio original came with the car.
The only true way is to find the build sheet, but different antennas went with different radios and that can tell its own story.
Joe
creator of Team Chevelle's radio info pages...slowly finishing them up...