That rpm that Adam is talking about is fan speed not crank speed. There are a lot of engines that the water pump pulley is smaller than the crank so it is very easy to overspeed the fan. Flex fan = run, run very far way.
So I should shelf the flex fan on my 340? I knew they were an outdated design and I figured the clutch fan was the way to go I had no idea they blow up and screw your **** up