I want to get really close behind someone driving slow in the fast lane
Are you in a hurry to get somewhere?
No, I want to scare him.
It is a crime to drive unsafely and harass other drivers. You could cause a major accident and hurt others or kill them.
That is not my problem. Let the driver handle his own driving.
You would be at fault in this case, since you were the one being wreckless. You should work on being patient when you are behind slow drivers.