Pull the plug on the tank sending unit and short the 2 pins together with a paper clip. With the key on, the needle should go to full. If it does, tour sending unit in the tank is bad, or maybe just the float is defective. If the gauge still stays on empty, most likely cause is the voltage regulator on the back of the dash pod, or the gauge its self. If you have a 70, I believe they only have 1 wire on the sending unit. If so just remove it and short it to a good ground.
