Stormy, you can get a master cylinder for a 67 mustang and bolt it right on the car. You will need a couple of reducing fittings that most any good auto store will have. I just converted a 65 and my 66 over - my brakes on both of these cars are power - You will need a low profile booster if you want to go with power drum brakes using a 67 master cyl.
National parts depot has these for 249.00. You can get a master cylinder for 20 bucks rebuilt about anywhere. All I did was remove the line from the dist block on the inner fender, put a plug in it and bent a new line for the front brakes. Simple job if you are a decent mechanic. I can send you photos of mine if you like. Hope this helps.
If you convert drum brakes to the dual master cyl don't forget the combination valve too otherwise you're wasting your time. That valve is a shuttle that blocks off a failed half of your brakes allowing the remaining half to stop the car. I'm NOT refering to a proportioning valve which is used on a disc brake setup.