Got Shell 93 Octane and now barely idles 1973 Dart 340

Yes it absolutely will; the booster vacuum IS the intake manifold vacuum. A leak at the booster is a leak into the manifold. With the booster line disconnected and plugged, a hard pedal is normal... YOU have become the power booster LOL.

The timing sounds OK at just over 10 degrees; maybe not optimum, but at least not in an unsafe range.

So did you adjust the idle mixture screws while looking for maximum vacuum readings? You generally have to go back and forth and back and forth on each side a few times.