Fuel sending unit?

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I bought a brand new 21 gallon fuel tank and matching sending unit for my 69 blazer from brothers truck parts and I have a new cluster from lmc. The issue I am having is I can fill the tank full of fuel and the gauge will read exactly full and it will go down just fine but when the fuel gauge is showing empty it will only hold about 14 gallons to fill it back up so something is not correct. Has anyone had this issue or does anyone know what could possibly be wrong?