what you offer him depends on what you find when you inspect the car. I looked at a 68 coupe, asking price $6K; it was a total POS and the guy was offering it to me for $3K by the time I left.
If it really is "GOOD," why don't you start out at $1K less than what he's asking.
If the car has problems, but you're still interested in it, don't be afraid to really lowball the guy...you might be surprised at how cheap you can get it.
It's hard to recommend a $ figure, because I have no idea what Stangs go for in your area, how badly you want the car, how much you're willing to spend, etc.
Good luck, and keep us posted.