Buy a new repro glove box housing, then chop to fit your stereo CD player in.
You can keep the upper dash stereo original, nobody will see it in the glovebox, and you can control remotely.
Budda bing!
The other option we've tried is our MP3 Player and a frequency sender unit ( cheap, about 20 bucks) that sends the MP3 songs to the original AM/FM radio in the upper dash. This works fine unless there is alot of local FM radio interference.
'68 428 HO M3 Monster, 4-on-the-floor! Need I say more?