I suspect a sinking float but no matter what you do the sending unit will have to be removed. pop the float off and shake it to see if it has any fuel in it. If it does you have the choice of a brass stock float or a plastic Ford#COAZ-9202-B
If that's OK check the resistor with an ohmeter. 10 full,70 empty. Adjusting this is beyond my ability but a thread ran a while ago about grounding the resistor to the body of the sending unit.
Hope this helps,
Ron

In sixty-five I was seventeen and running up one-o-one I don't know where I'm running now, I'm just running on Jackson Browne-Running On Empty