I am about to replace the sending unit in my 65 chevelle. Should I fill the gas tank or drain it before doing this? I don't want to blow up the car and myself. A mechanic friend of mine told me to fill the gas tank with water? That sounds insane. Should I drop the tank and completely drain it or am I being paranoid? If my car was originally a six cylinder, that would be a 5/16 sending unit right?