I would start with 3500 cash. Thats about what i got mine for and i had way less miles. Maybe even offer 3400, you never know until you try. Haggle with them, and don't be intimidated. They do it for a living, where i'm sure your a first time dealer since its your first stang. Make sure they see the cash, its like buck feaver, they get excited and just might take it, as soon as they do, take the car and run like a madman.
. I've bought about 6 cars and have got some pretty outstanding deals. Don't be the nice guy, i found that out the hard way.
Good luck