I just went through this on my 72 rr GTX. I wanted to keep the original look. I kept the stock AM radio in place and installed a Hideaway Radio made by Secretaudio SST.
http://www.casmfg.com/Secretaudio.htm
It consists of a receiver that can basically be put anywhere in your car (glovebox, trunk, under seat), a small LCD display, and an RF remote. The RF remote is the key because since it operates via radio frequency, you dont have to worry about pointing your remote directly at the receiver (which would be an issue if mounted in glove box).
The receiver and LCD screen are mounted under the drivers seat in my car. I actually installed the screen upside down incase I need to see the screen I can look at it between my legs. Also has an Ipod cord which runs under my seat as well. They have 2 versions, one with CD changer and one without. I got the one without the CD changer because I honestly dont own CDs anymore, it ran me about $300. I couldnt be happier, nobody would ever know its there and preserved the stock look.
For speakers, I went with a new 4x10 with 2 built in tweeters for the front dash, and 2 6x9s under the rear package tray. Mounting the 6x9s was a pain because my car has factory rear defrost, so the blower was mounted where one of the 6x9s should be. I used long bolts to mount the speaker and just put the blower below the speaker using those same bolts.
It sounds pretty darn good to me. Obviously you arent going to have significant bass because of the lack of a subwoofer, but overall sound quality is great. I still love the sound of my 440 anyways! I have pictures I will try and post at some piont this week.
