Sounds like a great deal, but...
There is probably a good reason you got the car for $600. If I were you, before I would start sinking money into the car I would do a thorough evaluation of the condition of the car. The biggest thing to worry about is rust. Compared to rust repair all mechanical work is cheap. There was a recent artilce in one of the Mustang mags on common rust areas. You can also search this site and others, such as stangnet and vintage mustangs
http://users.ntplx.net/~link/VMF/ for ideas on how to inspect the car.
I definitely would not drive the car too far without a comprehensive evaluation of the brakes. HTH