Fuel shutoff valve install

Looking for info on what size valves I need to shut off one fuel tank. Currently have a 60 gallon and 100 gallon. Would like the shut off, not disconnect the 100 gallon tank. My work does not always call for more then 60 a day and every little bit of weight I can save the better. Just want to install two valves I can turn on and off as needed. Current setup throws me off on how much fuel I have as it reads from the smaller tank.

Truck is a 2004 freightliner columbia