I have a 57 Chevy Belair and have installed 70s Chevelle power disc on the front and matching master cylinder for front disc/rear drum brakes. I want to put a 1980 Pontiac Firebird 10-bolt posi, disc brake rear end on, and was wondering whether to go with a 4whl disc Firebird master cylinder and proportioning valve or should I use an adjustable proportioning valve instead? I already have a power booster unit from an 80s Citation. I'd appriciate any input. Thanks.