Where is the fuse for the gas gauge?
There is no separate fuse for just the gas gauge. Is it actually reading full or is the needle way past the full mark? If it is past the full mark that is an indication of an open circuit, usually it's the sender but at this age it can also be wiring problems. Try disconnecting the sender at the tank if you can reach it and put a test light or a 1/4 watt resistor on the sender wire and ground and see if the needle changes position, if it does then it is for sure the sender, if not then you will have to start a search for a broken circuit.
Could be the float inside the tank
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