Should vacuum be on spark adv all time 1979 5.0 engine?

No, vacuum advance on a 1979 5.0L engine should not be on all the time. The vacuum advance mechanism is designed to add spark advance at lower engine speeds and lighter loads, improving fuel economy and drivability. At higher RPMs and heavier loads, the mechanical advance takes over, and the vacuum advance should be minimal or non-existent. If the vacuum advance is always on, it can cause detonation (pinging) and potential engine damage, especially under heavy load. The vacuum advance is supposed to be controlled by manifold vacuum.