Yes, a 4runner booster will bolt into your 73. As mentioned previously, you'll need an external inline check valve. I recently saw a neat little one in an 86 Nissan Pulsar (likely also in a Sentra).
I'm running an 80 series booster, 4runner, pickup, 60, 70, and who knows what other Toyota boosters will work. With the 80 booster (and master) it's almost like driving a truck with air brakes. The brake pedal is now like pushing agains a spring, the further it goes down, the greater the braking force... but the force to push the pedal doesn't increase noticeably. Unlike before where you had to stand on them with all your weight to stop quickly.
