1978 Pontiac Bonneville Fuel gauge

I have a 1978 Bonneville and my fuel gauge isn't working. I had bought it from a elderly women that didn't do nothing to the car beside put gas in it and get the oil change. So far I asked around and I been told two things either its the instrument cluster since my gauge and my gear shift notifier (the indicator that tells if the car is in park, reverse, neutral or drive) don't move/work or it's can be a bad sending unit in the fuel tank. I wanna know what it most likely the problem it is since I'm fixing up the car and etc. I'm leaning to and hoping its just the instrument cluster since its a old car and the newest thing in it is the radio I just installed and it has it original parts and I heard back then a lot of Gm parts malfunctioned like this a lot. Looking for all input.