You can tee off from the PCV hose, assuming you have a PCV valve and not a restricted fitting on the manifold. Most parts stores should have stepped tee connectors -- you just cut off the steps that are too small for the larger hoses.

There's a lot of mythology about "ported" vacuum (connected outboard of the throttle) vs. "manifold" vacuum (connected inboard of the throttle). Fact is, once the throttle is barely cracked open, they both work identically. There's nothing wrong with the distributor seeing full vacuum with the throttle closed -- just set the timing with the vacuum disconnected and plugged, hook it back up and reduce the idle speed to normal. Timing will actually retard as you first open the throttle, which is what's supposed to happen -- it just seems wrong until you get your mind around it.

If you happen to be installing the 123 ignition, I believe the vacuum function is locked out at idle speeds anyway, so it works like "ported" vacuum no matter where you hook it up to.

If it's from a '75 B20, it'll be a vacuum retard unit, which you probably don't want, so simply put a rubber cap over the spigot on the vacuum unit.

Cheers,
--
Barry '67 122S "Betty", '69 122S "Veronica"