Quote: I assume this is a SB? You can buy water pumps with a threaded fitting rather than the cast nipple. Then you use a pipe plug and that gives you a very secure block off.
If you stay with the water pump housing with the cast in nipple then I'd recommend a bypass hose rather than rubber cap. Just put the fitting back into the intake manifold and run a short loop of heater hose.
There are lots of ways to do this, just depends on how much "hillbilly" blood you have in you!
