You really need to drop the tank, or you are wasting your time.
Drop it, throw in a few small to medium size stones, a small piece of small chain etc., pour in some non flammable solvent and have someone help you shake it around, rinse repeat.
This will knock loose any scaly buildup and clean it fairly well. Install it back and put in a new sending unit WITH ground strap and you will have many smiles per gallon.
Of course this is dependent on the external condition of the tank, if there are any signs of rust build up to the point of breaking through then replace the tank, they aren't that expensive.