Easy. I'd get a pair of repro quarter trim panels, cut speaker holes, and upholster them. Back in the day, we'd cut these all the time, but now you'd want to preserve originals.
5x7 would be no problem, and even 6x9 would work if you were careful. Get flat steel grilles, and paint them to match the upholstery.
Run the rear channels of the radio to them. Use a Dual Voice Coil 4x10 in the dash, with the front channels.
For the radio, no question, I'd use the USA-66 radio, which is a repro of the 65-66 AM/FM radio. Some people talk about problems with Custom Autosound, but I've had good experience with them. You can even connect a remote CD changer to it, operated by the pushbuttons, or use the aux inputs for mp3 player, etc.