Twin-shaft OEM and aftermarket radios were common upgrades for American cars back when they were relatively new. However, new, commercially available twin-shaft radios today are limited in availability and quality. Often (at least for older Mopars), twin shaft head units don't fit well in the OEM cut-outs and dashboard modification is necessary to install them. New twin-shaft options are still available from:
Many radio repair shops offer FM conversions and this is done with the Aurora Design FMR-1 AM/FM Receiver or the Aurora FMC-1 FM Tuner. The FMR-1 will convert your head unit into a modern 180 W multi-speaker AM/FM radio with auxiliary input but requires the gutting of the original internal components. The FMC-1 adds an FM tuner and auxiliary input to an OEM radio by splicing the FMC board between the antenna lead and the radio circuitry but does not change the audio output (number of speakers or power). To cycle between AM, FM, and auxiliary input with either system involves switching the radio off and on.
Another option is to install an auxiliary input radio adapter like Brew City Engineering's RediRad. This device connects between the antenna lead and the radio and allows you to add the input from any device with a 3.5 mm headphone jack. The RediRad adapter "broadcasts" into the antenna lead at 1000 kHz so you need to tune to this frequency on your radio. When the RediRad adapter senses output from your portable device (smart phone, mp3 player, etc), it will switch over the 1000 kHz signal to your device and will switch back to the over-the-air broadcast signal within a minute after the device is turned off. Since many portable devices have FM tuner capability when used with head phones (the headphone cable becomes the FM antenna), the RediRad adapter can also add FM to your AM radio.
With the car's body being a huge Faraday Cage, the best place for an FM antenna is outside the car.