Vacuum is one indicator of the health of the engine. The more vacuum, the better your air pump is. 15” of vacuum for a stock grind cam is an indication of a bad tune, open vacuum source or mechanical failure such as a blown gasket, bad rings etc.
Hook up a vacuum gauge to a direct intake port. Plug off all other sources including the vac advance on the distributor (diaphragm can be blown and leak) and brake booster. Take a vacuum reading. If its still at 15”, and you’re certain your ignition timing is correct, then you have a bad tune (throw out the edelbrock) or a bad intake gasket etc.
This is on my next steps list. I left the advance hose, brake booster, and PCV valve hose all connected. I did use the intake port on the right side of the carb to check vacuum. Will try this and get a more accurate vacuum reading.