69 Barracuda heater control vacuum lines on firewall

I'm dealing with the same thing on my 70 Valiant.
The previous owner had no vacuum lines hooked up.
I identified the 2 lines to the water valve (My AC is gone too).
The third line, as i'm learning here, is the vacuum source. Mine has what appears to be a brass connector on the end. Did the factory vacuum reservoir have a connector like that?
It's obvious I need to connect my vacuum. Do I just pick an open port on the carb?
Thanks for letting slide into the conversation. :)
Danny