I recently picked up a 88 Camaro S/C with a bad 305 in it. The previous owner had started taking parts off the car in preparation of pulling the engine. I have been able to identify where most of the wiring and vacuum hoses go but there are to vaccum lines coming out of the firewall by the a/c. I believe that one of these is the line to the heater valve and the other is vacuum supply to the HVAC control but am not sure where it attaches to the engine. Either the fitting on the back of the manifold or on the throttle body.

 

You should be able to attach anywhere you desire that you are getting manifold vacuum at. There should be check valves installed (small and cheap) so that the vacuum isn't reversed during accelleration and change your heater output to defrost output (default setting when vacuum not present).