The new vac pot had both ports? I've only seen replacement pots with one.

Sounds to me like the vacuum retard port is hooked to the carb. That would
be the one pointing upward. In that case, the timing is being retarded
rather than advanced. The vacuum retard port was always routed via a vacuum  
sensor screwed into the thermostat housing.

It's also possible (but unlikely) that your breaker plate is seized.

There is a great deal of confusion about these ports. Vacuum advance is
always ported off the carb. I've had people argue me blue in the face
claiming that vacuum advance is taken from the intake manifold. Sorry, that
does exactly the opposite of what's intended.

It's also possible that your timing chain has jumped a link, and you are
adjusting the distributor to compensate. I've known engines to run quite
well like this until the timing chain jumps another link. BTW, 8 MPG is not
unlikely with a 390, depending on carb and other options. It's one of the
most inefficient engines ever made. It can be made efficient, but it takes
careful planning as far as carb, intake manifold, heads, valves, cam, and
CR.

IMO, the timing light should have been your first move.