I looked into it before buy the Pontiac am/fm unit from custom auto sound. the quotes for converting my original unit were very expensive in comparison. usually in the $600-$700 range, depending on what features you want to add. (Bluetooth, mps, etc) the custom auto sound unit was less than $300.
I met a guy at the last Coker Tire cruise in event. His name was Allen Forgey at Old Car Radio 423-344-8357. they take your stock unit and upgrade it for FM, Bluetooth, Mp3...even buff out the chrome if is pitted