most of the cars i see have this problem , i have restored three b bodys and all do the same thing 5/16 , 3/8 does not matter .
i have replace the wire from the back of the guage cluster to the sending unit , made sure the sending unit is grounded and changed floats to brass from plastic .
this all had no effect on the guage , reading !
Here is the weird part , if you hold the dang thing in your hand with all the connections made i can move the float by hand and make it go full and to half when out of the tank . Unbeliveable right I know!
my take on this is some thing has been lost from the factory units to the aftermarket , either wrong calibration OR the float hits the top of the tank when inside the tank so it will never read full .

someone said you have to bend the arm on the sender, i don't know if this works or not . might be worth a try bending it down so it will rise higher in the tank .