The highest the fuel gauge on my 77 will register is 1/4 full.  As I drive it will gradually go down to E.  Is there any way to determine if the problem is the gauge or the tank sending unit?  If I drain the tank, can I see the sending unit from the filler opening to detemine if something is wrong in the tank?  The previous owner (owned it about a year) said the problem was there when he bought the car, and he just lived with it.  He said the owner previous to him had a lot of restoration work done and maybe that's when something got screwed up. 

The gauge is the car is controlled by the sending unit grounding the gauge unit.  In our cars the test is not difficult, once you reach the wires.   Unplug the wire connection to the tank.  Turn the key on.  The gas gauge should go up past the full mark.  Now ground the wire from the gauge.  The gauge should go to empty.  If you put a resistance in the wire to ground it will read somewhere in the middle.  If it does that, the sending unit is the problem.  If it does not, the problem is in the gauge.  Before replacing either one, make sure the wires is okay and not partly shorted/grounded.  The will make the gauge read low.  Corrosion is a common cause of a short to ground that can cause this problem.

It's possible that the fuel tank bladder has become deformed, and the float is hanging on it.
If you don't find a problem after doing the test that Ken suggested, you'll be diggin into the tank...the sending unit, if it is damaged on the float contact, will normally only mess up in one area, say, 1/2 tank. It will read "Full", and go down as you drive, until it gets to the 1/2 area, then it may drop to "E", or it may go all the way back up to full, until it gets past that one spot. Then it will start working correctly again.
Unplug it, and it will go to "Full", ground it, and it will go to "E"....Ken said that already, didn't he?
Chances are, the bladder is hanging the float...

Joels right.  And if the bladder is hanging up by collasping inward, check the charcoal cannister vents.  When they plug, the bladder collaspes inward.