A low steady reading on the vacuum guage would indicate either a slight vacuum leak, retarded valve timing, or ignition timing set to low. A big cam would bring in a low reading but the engine would still run OK. I would think that 12-15 degrees ignition timing would be in the ball park. Obviously 40 is not correct. Incorrect valve timing could explain the black plugs. And a vacuum leak could explain the low vacuum reading and rough running engine. Since the needle is steady there doesn't appear to be anything wrong with the valves themselves......Just my 2 cents.