It is possible to use a reasonably small vacuum brake booster beneath the floorboards in the original position operated by the stock brake pedal. You would have to either buy a mounting bracket or fab one up for yourself. You may as well put a double-unit master cylinder behind it while you're at it. It's a simple matter to run a vacuum line forward to the engine.
I used an 8-inch booster in my rig. They make even smaller diameter ones, however. Willwood makes hydraulic fluid pressure retention units for either drums or discs.
