Have a 1986 chevy pu, pulled 305 out about ten years ago and installed a rebuilt 1969 350 two bolt junkyard motor.  It has been sitting up for five years and I am restarting my project.  Having problems with the manifold vacuum.  Three lines come off: one to accumulator, one to transmission cutout switch and the other is my question.  All diagrams(Engine, Chiltons and Haynes) are not any help.  Please help. I will be back with electrical questions in the future while I restart this ten year project.  

You should have a vacuum line going to the power brake booster, that's probably what it's for.

If that's already hooked up, it could be the vacuum line going to the distributor.