There are baffles in the tanks to help minimize the sloshing & associated excess gauge movement. At just 1/3 tank you won't get much benefit from the baffles. These old gauges work in real time unlike modern gauges. I would guess that what you are seeing is normal for your truck.

Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.