Here's a pic of the oil cooler housing, courtesy of Cranetruck. If this is what you are talking about, it is for oil sampling.
I would open the vent on the heater, then fill until coolant starts coming out, close the vent and top off. Get it to operating temp and let it cool, then verify coolant level.
I bought an Airlift brand coolant filler years ago, and it does a great job at ensuring no air pockets, even in some of the newer cars with coolant systems that are prone to air pockets. You pull a vacuum on the system using an air hose through a venturi. Then you shut off the system, watch the gauge to ensure no bleedoff of vac, then attach a filler hose and stick it in a bucket of premixed antifreeze. Open the valve and let the vac in the system draw it out of the bucket.