heater by-pass

I want to install a vaccum by-pass valve for the heater to cut off flow of hot water coming from water pump to the heater core. I've seen alot of cars with a manual shut-off valve spliced in, but I wanted to be correct in the restoration. I have the valve, but no info on where to connect the vaccum hose to once inside the firewall. Theres alot of vaccum hoses under the dash pad of my 72 SS. Where to I stick that hose?