65mustang347stroker

I have a 65 mustang and i havibg fuel gauge problems ,i just put a tail light wiring harness in it and check all wiring and it is correct ,and when i hook my fuel gauge up it goes pass full when .i got about 1/4 tank or so ,the gas tank is only 2 yrs old so is the send unit .i had the gauge hooked up on time and it read fine.(it a project car thats why i only had it hooked up once)i got two other 65 mustang parts car so i tried the constant voltage unit but it still does ,i guess i will buy a new one to see if that the problem .anyone know?