While I had my car apart to install an heat / AC unit, I noticed that the radiator water was cruddy brown. The car had a rebuilt engine that was only a couple of years old. After trying unsuccessfully to buy a new radiator (the people I spoke to said that it had been custom made and the core was taller than a Mustang radiator) -- I took the radiator to have it rodded out and cleaned. 

The guy said that the radiator was horribly dirty and was very hard to clean. I was surprised because the receipts that I got with the car showed it to have been re-cored in 1997. The radiator guy said to remove the lower hose at the water pump and be sure there was not any crud or wires (from those accordion type universal hoses) . Below is what I found when I removed the lower hose.

The pump that I got looks like the web page picture, not the one above that was on the box sticker.

So far, all I can say is that the pump is installed, and fits. The car had only been overheating a little bit before, so it ought to run great.  Before buying one, be sure to check with me if I have not come back and reported here after putting some miles on the new pump. The cost, with "free" postage, but a $9 handling charge, came close to $100.

As for me replacing a water pump (I am NOT a mechanic), nearly every part of the car hangs off that water pump, so I made extensive notes as I removed stuff, and drew pictures of each belt configuration, and where there were washers used as shims. I labeled every nut and bolt, and made notes as to the order that they were on the car. "Bolt head at the front of the car, then lock washer, then through the idler assembly, then 2 washers then into the water pump" for example.

The old water pump is for sale, cheap.  Ha ha.

Could THIS account for some of the overheating problems we are seeing?

FAN SHROUD --(added to my 1978)

As part of the air conditioning conversion that I did, my mechanic said that I could use MORE AIR going through the radiator and A/C condenser, instead of the fan pulling some air from AROUND the radiator, or over the top of it. He suggested moving the radiator a bit closer to the fan, which I did with some spacer bolts, and he suggested a fan shroud.

I bought a 3" standard, universal chrome shroud from eBay and installed it by making some C shaped adapters to bring out the mounting spot to the side of the radiator. These are supposed to go on wider radiators, using long plastic nuts and bolts that are pushed through the fins of the radiator. These are generally included with the shroud.  In our case, the shroud would hit the fan since the fan blade is as wide as the radiator core. So I made mine a bit more oval, using the adapters to spread it wide enough to clear. Since there are four mounting spots for the shroud, I used the two side ones and one of the upper ones which I used the plastic through-the-radiator mount. I couldn't the other upper bolt through the radiator and I had visions of busting a hole in the radiator.  The three spots hold it firmly in place.

Above is the C shaped adapter I made out of scrap aluminum bar to move the shroud out wider than the radiator (passenger side view). I had to drill new holes in the mounting ears on the sides of the radiator. The extra nuts you see up and down the sides of the radiator are where I tried to move the radiator to the rear of the mounting posts. One nut as a spacer at the bottom, and two at the top. The radiator was not evenly spaced away from the fan at the start. I used the old short bolts that held the radiator to connect the ear on the shroud to the adapter.

The mechanic said that the A/C high side went from 380 or so pounds down closer to 240 pounds. With my new water pump, the car seems to run the same either way, although I have not tried it on long trips.

Here is a view of the shroud in place. Because of replacing the radiator and A/C parts, I had the radiator shell off the car. I will be painting over the rust on the radiator - - again!