what does the vacuum advance do? and how important is it? i tried to blow air through mine but its plugged and the hose coming off of it is plugged into the intake, is that were it should be plugged into or should it be off the carb?

 

What motor? Your stock 305 in that car shouldn't have vac advance on the dist, but assuming it's been replaced...
It gives better efficiency on the highway. So more MPG, better combustion (less plug fouling) etc.
Pretty important, unless it's a race car, it should have it.
It should be plugged into a "ported" port on the carb, then to the vac advance can.

The vac advance can isn't something you can suck on and see if it works.... Well unless you're the type who can "suck the chrome off a bumper" it's a waste of time.

If you're plugged into the intake manifold instead of the carb, you'll have lots of ignition timing at idle. This may be a good thing, most of the times, not.