The water pump is driven by the timing belt on the 3.5. I attached a picture of the pump and the location below. Sorry. I know you just had the heads off too, and you going to have to remove the timing belt again.
Overheating on slow downs scream air flow. You need to check if the radiator is getting the air pulled through it when stopped. Verify that the fan is operating correctly. I will assume that this does not happen on start up, only after the vehicle reached operating temp?
Also, check for any external blockage of the radiator, bird nests or mouse houses between radiator and condenser. Also, when the vehicle is at operating temp, shut it down, and see if you can feel cold spots on the radiator. That would indicate a blockage in the cooling fins. I use a laser thermometer.