1968 Dart Brake and Booster issues?
I'd like to rule out the Distribution block as the source of the problem before going on to tackling the quite possibly leaking Vacuum Brake Booster.
The sound that your booster is making is normal.
Your booster has two separate chambers in it; the one side is under vacuum, while the other is at atmospheric pressure.In between them is a valve system.
When you step on the brake pedal, the linkage opens the valve and the diaphragm pushes on the pistons in the M/C in direct proportion to how hard you are pushing but amplifying your power by about 7times(IIRC). The sound you hear is atmosphere rushing in there to do this work. And rushing out when it has finished. There is supposed to be a foam-rubber muffler on that port so you don't hear it, or at least don't notice it.
To test your booster; there are a few tests.
Test-1)
With the engine off, equalize the chamber to atmospheric by pumping the pedal about 5 times. Then step on the pedal with about normal power and hold it right there, then start the car. Immediately on a stock 318, the pedal should rise and force your foot upwards. That is normal.
Test-2) idle drop
Start the engine and wait until it is idling normally. Watch your tach. step on the pedal and hold it there. You might notice a slight dip in rpm followed immediately by a return to normal. Try it several times, with varying pedal pressure, allowing a couple of seconds between each application for recharging. If it passes this test, there is nothing wrong with it.
Test-3) vacuum decay.
start the car and let it evacuate the booster for a few seconds. With a big cammed engine, to at most 10 seconds with the engine at hi-idle. Do not touch the brake pedal. Turn off the engine.
Come back in ten minutes. Put some light pressure on the pedal than start the engine. The pedal should rise just a lil under your foot, but not like when you emptied it before. If it does as before, then the atmospheric valve may be leaking but more likely is the charge line under the hood.
Test-4) This is more difficult to do accurately. You need to Tee a vacuum gauge into the charge line between the booster and the check-valve so that you can see what is going on inside the booster. You need a fitting etc, but this is a last resort test.