Finding a 68 AM FM stereo will be difficult...very rare option. Did yours come with the AM FM originally? If you are looking to have a period correct radio in the dash, finding an AM only radio would be much easier. If you want something more for listening or sound quality, such as AM FM Stereo, Auxillary input, Bluetooth etc, these are other things that can be done using the stock AM radio. They are not Concours Correct but those ideas can be discussed in Private Messaging if you like.
Richard