Regular Maintenance Is Key
Even if you want to keep your 1971 Mustang stock, you should still be regularly checking your pads and rotors for disc brakes or drums and shoes for drum brakes. These items are designed to wear down over time, and need regular replacement in order to maintain their functionality.
You can also look into getting an upgraded set of brake pads or shoes. These will act like stock replacements, with no modifications necessary, but will improve your stopping power.
For added stopping power, upgrading your rear wheels to a set of disc brakes is a great way to add stopping power. With a four-wheel disc brake setup, you'll be in a much better position to stop quickly and safely.
Some owners even opt to upgrade their 1971 Mustang with a big brake kit. These kits are designed to brake even in performance applications. You'll get added surface area for more grip and stopping power, but also often added ways of dispersing heat to reduce brake fade.
Not the year you are looking for? Check out these years: 1965, 1966, 1967, 1968, 1969, 1970, 1972, or 1973 for additional options.