Where do people hook the modulator valve vaccum line up at normally?

Full time vacuum, below the throttle plates, manifold vacuum.

What about those of us who have the vac. advance plugged into the full time vac. port on the carb?

Install a "T"

You can also install it into the top of the intake manifold. Most manifolds have a threaded hole behind the carb on the passenger side. The factory uses a threaded fitting that has either 1, 2, or 3 smaller nipples on it that are used for accessories or the transmission.

a T works too.