you should not have to take the head off either. to replace a spring you can use a compression gauge hose and put shop air to it and install it where the spark plug goes. you then use a spring compressor to remove the valve keepers and then you can remove the spring. The air keeps the valve from falling into the eng. depending on the cyl it should not take long only a couple of hours.
it is a tedious job with the motor in the car but can be done with no problems. you will need a special valve spring tool if working on the 4.6. it will take you a good 3 hours time you tear it apart and put it back together even longer if you dont have much knowledge of it.
I agree with all of the above, it can be done, but it may also depend on why you want to change the springs... if you are upgrading, doing it with the heads on should be ok, if you had a failure, might be much better to pull the heads at least.
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