When i replaced the stereo in my 7Litre i made a wood mount that bolted to the original hole from underneath and put two 3" speakers into it. That way i could actually hook up a stereo and listen to music. Unless you are only going to listen to AM talk radio that would be my advice. i think someone might sell this set up now. Check with AutoKrafters or Dearborn Classics or whoever that company is that makes the repro 3-hole radios for Mustangs, etc..
As far as the stereo i went non original. I bought a piece of thin plexiglass and cut out a shape to match the stock cluster bezel, then cut a hole for a modern 1-DIN stereo. The stereo itself fit into the old radio hole. I think i may have had to fab a surround for the hole so it looks stock. Also, i didn't want to cut any of the original pieces so i bought whatever i needed to modify from a an old 500 in a junkyard and put the original bits away. I did this 10 years ago so the details are a bit fuzzy and am out of the country so can't go look for you.
If you want to stay mostly original you can use the same idea with a single speaker. Get whatever replacement speaker you can find from Autozone or whereever and just build a thin adapter out of plywood or masonite to make it work. Anything will be way better than what was in there.
