When I installed the 408 in my mustang I decided I did not want to hook up the heater hoses for a couple of different reasons. The car is only summer driven so I don't need any heat and I was also afraid of blowing the heater core. I have heard this sometimes happens when turning the water pump at high rpms and building excess pressure. The engine has solid lifters so I would be removing the valve covers occasionally to adjust the valvetrain and it is nice to not have to deal with the hoses laying in the way. I also love the clean look in the engine bay without the clutter of the hoses.
I originally got a small rubber cap from a rad shop. It installed on the nipple from the water pump and it worked well. However, I have also heard these aren't the best quality rubber and should be replaced every couple of years or so.
My remanufactured water pump gave out on me so I ordered up a new aluminium pump from Summit.
I was thinking about trying to thread the nipple in the old cast iron water pump or installing some sort of plug in it to replace the rubber cap. After looking over my new milodon aluminium water pump I decided to give it a try. As it turned out, I was very lucky and a 1/2 inch tap fit almost perfectly into the nipple on the pump.
I turned the pump upside down to keep the shavings out of the pump and threaded the nipple. The aluminium threaded very easily.
I went to my local industrial fastener place and got a 1/2 inch fine thread bolt. It also has an allen key type head on it for a cleaner look and a hose could be installed over top of it in case it did start leaking some day.
The inside diameter of the nipple was ever so slightly bigger than the size of the tap drill recommended for the 1/2 inch tap so I added some thread sealant onto the threads of the bolt for a nice watertight seal.
A small shot of aluminium paint on the head of the bolt would make it all but disappear. I forgot so I will go back and touch it up with a brush later.