A lot will depend on your year and model but assuming it's a late 60's-early 70's setup and you have a water pump housing with 4 holes on top, the hole closest to the water pump is the return hose from the heater core. That also should be the hole nearest the centerline of the engine. The hole closest to the passenger fender (next to the valve cover) is the output to the heater core and would the one you should attach to the water valve if you have one in your system. The holes inbetween the heater inlet and outlet are for sensor, senders, and smog valves. The standard temp gauge sender should go into a small 5th hole pointing to the front of the car. Some of the C-bodies used a different temp sender that went up top in one of the big holes. Just put pipe plugs into the ones you're not using.
