If I were in High School, I would get a modern stereo and mount it in the glovebox. Look up the replacement "kick panels" that have places for speakers. There are also package trays (the flat area behind the back seat, by the back glass assuming you have a coupe) that can take two 6x9's. Some of the old looking but modern internal head units are pretty pricey.
What is your budget?
PS - Resist cutting up your dash to install it, you'll probabaly hate yourself for it later.
Another idea would be to build a console out of plywood and cover it in black carpet, mount the modern DIN unit in that below your old radio....
This an older in-progress picture of the now outdated Kenwood Excelon I have in my '67. The face rolls completely over to reveal the controls and CD slot. I've always liked the blank face, it's a different and low key when turned off. I found a stereo bezel that had already been hacked somewhat by somebody else and just cut it out some more for the DIN radio. A bit tight but I believe you could pretty much fit any single DIN head unit you like. One with a flip-out LCD screen might be a problem though. One with more rounded corners on the faceplate would actually be a better fit than mine.
yea they have the one next to the defrosters in the dash,2 more for each kick panel, 2 in package tray, backseat driver: 2 10" subs wth a BOSS amp. that is about as FAR as i would go. umm i was thinking about later on trying the backseat driver. you can find it on most mustang parts site. It has 2 10" subs with BOSS amp and it is installed on a board that divided the seat and the trunk seat divideder (only works on coupe). and if you are missing your seat divider might as well right.
Coupe, fastback or vert? Coupe is easiest. If coupe you can modify the radio bezel (the chrome thing) to allow a DIN receiver. Put a pair of decent 6x9 speakers on the package shelf (cut to fit the 6x9s...I think the steel under the shelf has 6x9 holes).
Try Amazon, New Egg, ebay etc. Might even take a look at Crutchfield.com.
'67's have three holes in the metal package shelf. One in the center (where '68's got their optional rear defrosters installed I believe) and two regular speaker holes. Unfortunately the holes are a touch small to flush mount 6x9's in. You can use spacers to raise them a bit and they'll work. Or do like I did and hang them underneath for a stealth look. The 5x7/6x8 chassis speakers should fit fine. In all cases you have to cut the trim board back there but a new one is only like $10. I just drilled a lot of holes in mine and covered it with some acoustically transparent material that matches black nylon plush carpet, even though it looks gray in pictures.
I have a DIN Kenwood in my 65's dash, since it was already hacked up. I have been considering a USA 66 or Retrosound unit. If I had a 67-68 I would probably cut a repop bezel for a DIN radio as long as it lights up green.
well they call me the breeze , i just keep blowing down the road ! i have a sanyo mar2280 in dash am/fm cassette that i pulled out of my last 65 b 4 i sold it and its going in my present 65. i have a pair of sony 6x9 speakers in those little wood boxes. oh and i bought that sanyo back in the mid 90's. everyhting i have is OLD even me !