in other words you want to upgrade the stock single master cylinder to a dual bowl master cylinder. there is no kit for this as such, but the conversion is easy enough. if you have power brakes already, then get a drum/drum power brakes master cylinder from a 67 mustang, and plumb it in. you might have to clearance the shock tower for it to fit properly.
if you dont have power brakes and want to add them, then there are conversion plans and parts available from many places, and you have many choices. for instance mustang steve uses and geo metro(?) brake booster. npd sells stock boosters. either way you are going to have to relocate the pin on the pedal that allows you to actuate the master cylinder, and mustang steve has the necessary parts and plans to do that as well.