I have a 68 Dart and the fuel guage works only when it wants to.
It will ususally work after the car has been driven for a few miles but
the other day it refused to work at all even after putting 5 gallons in the
tank.

I had a 77 Aspen that did the same thing but always started working
after a few miles.

Is this problem in the tank sending unit or the dash receving guage?
Or is it a wiring connection problem?

Does the engine temperature gauge also misbehave? If so, the instrument cluster voltage regulator is the primary suspect.

The temp guage is fine

OK, well the first thing to check is the connections at the dash circuit board. The gauge is secured to the board with two speed nuts. These can loosen and/or corrode. Go ahead and pull the instrument cluster and check out the attachment points and the main wire connections carefully. You can briefly apply a household 9-volt square battery across the fuel gauge posts to see if it moves. If it does, the fault is likely with the sender. New, 4051004 from teh dealer.

http://www.slantsix.com/UBB/Forum16/HTML/011061.html

check this test out for the fuel problems, it came from.com , looks to be striaght forward

The fuel gauge on my 1975 Valiant misbehaved like that. Finally removed the sender and found a spotweld had let go and things were bent out of shape. Bent it back to proper shape and soldered it back together.
It lasted many years until the car rusted away and fell apart. I still have the sender as a spare for my 1976 Valiant, somewhere hope.

My valiant did the same thing, the guage would only read 3/4 when full and past E with about 5 gallons in it. I eventually jacked up the car, removed the sending unit and saw that there was a break in the wires. I didnt want to try to saulder it back because I'm not sure if that would have an effect on the resistance so i pocketed another working one from a local pick-a-part