Thats wierd, I had the outie 5/16, I used a baby adjustable wrench, came right out. Stock Motorcraft waterpump here still, knock on wood.

 

I'm not sure if I have a stock waterpump or not.

 

My pump was original when I needed to change it. It had a plug with a recessed square hole, which is what caused me to make the tool you see above.

The new pump did not have anything in that hole, so I made the tool to move the plug from the old pump to the new. I later exchanged it for a hose fitting when I added coolant filtration.

That photo is in one of my galleries, and it's come in handy, as I've posted it a few times.

Pop

 

Well, that I guess resolves that issue. I'll just make a square tool and be done with it. End of story.