It could be a faulty gauge, or it could be a faulty float valve, since it goes up half way it's unlikely to be the link between the front and back of the car
The float valve is attached to the fuel sender unit, it could be a sign that it's a little gunked up in there which may be a good time to change the sock fuel filter too and give it all a good clean
Are you sure the tank is full? sometimes you have to put the fuel in slow with these old cars because the petrol pump clicks off too soon in the thin filler neck
