Those base plate adapters (Holley square pattern to Q-Jet pattern) are notoriously bad for gasket surface, and have almost no contact in a few small areas. I'd "paint" it with silicone temporarily as a test rather than continue to re-start/stall - you can clean it up if you find it. You can even use grease, anything viscose enough to seal a leak but not volatile.
Is the vacuum advance connected to manifold vacuum (below the throttle body) or venturi vacuum (above the throttle body)?
I prefer venturi (ported) vacuum, but disconnect and plug to be sure the advance isn't affecting operation and remove diaphragm leak as a cause.
Are you sure the power brake diaphragm is intact? This will have 2 effects if it's leaking:
1. obviously, it allows air to enter
2. it also triggers the PCV to go to WOT mode, increasing the leak volume
If it's even suspect pinch off the line to the booster.
