1965 F100-Convert single circuit brakes to dual

Hi, Daily-driver 1965 F100 Stepside. Anyone ever heard of changing just the master cylinder to dual circuit, and keeping original drum/drum brakes? Would it stop ok with the rear in case of front failure? Guess we could test this after upgrading. Would an adjustable proportioning valve help if dual master cylinder installed? Is there a bolt-on dual master cylinder available from another year or aftermarket? Most info found is for disc brake upgrade. Thanks for any ideas. Dave