The disc brake cars came with a dual diaphragm booster, as opposed to the drum brake booster which is a single diaphragm booster. What this equates to is more foot pedal pressure, and more stopping power. Yes you can use the drum brake booster, and MC, BUT in an emergency stop situation you may not have enough line pressure to lock up those calipers and rotors.
You can run a booster from a 73...if you have the room. They are generally a bigger diameter booster, but shallower from the firewall forward, as compared to the stock 68 disc brake booster. The cardone interchange will more then likely send you a booster from a mid 70's dodge van, or motor home. They have the same basic dimensions, and function the same as the 68 unit, but you might need to grind out the main hole in the firewall to clear the booster piston. I know this through experience....;)