67 Mustang brake problem

My 67 fastback with a lil 289 stock engine has manual brakes drum brakes & auto trans. To get better braking for small money, I would like to try adding a vacuum booster. My auto parts store has a booster that would fit with some mods to the fire wall. I see on the web that Master Power has a kit that may work fine too but they warn you on vacuum pressure needing to be a min. of 15 psi or so. Has anyone out there tried this or should I be doing something else? HELP!!!