The day after you changed the fan clutch it overheats, having not overheated before.
Then a new pump (with the suspect 'new' fan clutch?) is installed and it still overheats. "It looked like the fan clutch was not engaging, so the tech suggested a new radiator. I installed that, still no change!"
Hmmm, after it didn't look like the fan clutch was engaging and suggesting a new radiator, NOW the tech agrees to change the clutch but 'thinks' head gasket.
HOLD EVERYTHING RIGHT THERE!!!!!
First, have the tech explain how he will show you that the fan clutch is, or isn't, operating properly. Then, have him explain, slowly, how the radiator affects the clutch.
The clutch engages the fan to mover more air to cool the radiator. So, as the radiator heats up the clutch SHOULD engage. If it doesn't engage the fan can't cool the radiator. Why change the radiator???
If the fan and clutch are not operating properly have him change the clutch again at no charge. Repeat until YOU are satisfied that the fand and clutch are operating properly.
If this now keeps the car from overheating, tell him what he owes you for the water pump, radiator, brackets, etc..
If it still overheats, have him show you that the thermostat is NOT at fault. You didn't mention that it has been checked or changed.
A head gasket CAN result in overheating, and because I've had several let go in other cars I know that there are definite symptoms. Before you he even THINK about head gasket replacement insist that he show you some definite symptoms of head gasket failure such as bubbles in the coolant, excess coolant pressure without excessive temperature, or antifreeze in the oil. You might also want to get second opinion from another tech.
In my opinion he must do much more than just "think" it might be a head gasket. So far his "thinking" has been at your expense. If he has a problem explaining his diagnositic steps and reasoning in a way that you can understand, I'd suggest finding another technician for future work.