A few of us have done it with both drum and disc brake cars. If you search the forum for the term "servo" you will find at least two or three threads and a few photos. My preferred method is to add a remote booster to the front circuit only (if you have a dual circuit master cyinder), it seems to be the simplest way to do it, and was the way makers like Alfa Romeo etc beefed up their brakes when necessary. A lot of people with 60's muscle cars, rat rods etc use the same method.