When its in the car dont let the water hit the fins unless its low pressure/at a distance. If you want to thoroughly pressure wash the radiator you shoudl drain the cooling system and remove it from the car so you can do it properly by directing the water STRAIGHT THROUGH the fins so as not to flatten them.
Hit it straight and don't use too much pressure. I would use the 40* angle head. Never thought to use the pressure washer on it, makes sense, might try it.
just be careful with a lot of pressure, my cousin was trying to get bugs off of his bumper, and got too close and now he has a line of missing paint. A radiator fin is just as easy to bend.