Please don't cut the dash! There are other ways to get better tunes in your stang...solutions that are even better than a single din radio. I'd look into a modern unit that fits the dash from a place like Classic Car Audio. One with an iPod port. Then I'd wire it up so the ipod can connect in the glove compartment.
If you're really hell bent on cutting that hole, there really isn't a sure-fire way to do it with precision. If you're just a little off, the radio will be crooked or the part that holds the radio in will be loose. There also isn't a way to dress the cut properly. The curve of the dash prevents any modern trim plate from sitting flush. You will definitely see the cut from both sides.
The PO cut my dash and I wish I could've stopped him. My radio is tight in the slot, but, like I mentioned, I can see the cut where the dash curves.