[RICK LAKE]

Xack You want the metal disc to be the same size as the impellor. There are no holes in the impellor fins. You will need to drill them into the fins and disc. Drill in the middle of the widest part of the fin. Drill slow and take your time. You don't want to damage the bearing or seal on the pump shaft. Make sure you clean ALL the metal chips out of the water pump housing. Good steel pop rivets work fine. IMO, I wouldn't use aluminum ones.
The reason for this disc is to increase flow of the pump and not let coolant get around the impellor.
You didn't say what the temp on the thermostat was? You will see the temp numbers on the bottom.
I think this will bring down the temp about another 10 degrees. If you need to drop the temp more, we need to look at pulleys next. You may be over running yours too much. The coolant is not getting enough time in the radiator to cool down, again we are back to air flow in and out of the car hood. I have added 2 blower fans from the boating world to help remove the hot air from under the hood. They blow out the side vents in the fenders. Have to rap them in foam tape that heating and ac guys used to protect the plastic from melting. I also have a little extra shielding to protect them from header heat. I don't know what is the design of your sidepipes inside the housing but have a small inlet and outlet will increase the heat under the hood also. If you are worried about the temp of the motor sitting in traffic get a small piece of 2x4 rapped in cloth and open the backof the hood 2" and this will also help bring down temps. Rick L.