Hey:
The books that I have show two vac lines coming out of the firewall, the black one goes to manifold vac at the fitting behind the carb and the white line goes to the water valve.
Since you see only one line coming thru, you may have to remove the radio to see this, look under the dash at the AC control for a line that goes from the three fitting vac switch to the multiple fitting vac switch. There should be a tee in that line that goes thru the firewall, this is the vac line that goes to the intake. Look at the three fitting vac switch, the middle fitting should have a vac line with a white tracer. This line should also go thru the firewall to the water valve. Maybe it goes thru in a different place?
I had a 71 SS and it only had one line, if memory serves, and I do not remember what fed the water valve. However all of the books that I have still show two lines as above. Go figure.
Hope this helps
Ed