Well, Im still at it. I bought a new sending unit. All the grounds are good, and the gauge still doesnt read right. I bypassed the wire from the tank, and temporarily ran a new wire from the sending unit up to the back of the cluster. Im guessing the gauge is bad then? Its weird how it shoots to full when the sending unit wire is grounded though.

When you say it doesn't read right what is wrong?

You may have to bend the float arm down to make it read right.

The sending unit is brand new. I checked the movement of the float when I put it in. The old sending unit didnt read any differently

I bought a new 3/8 sender for my 68 charger last year. I don't know if the old fuel gage worked or not because I bought the car and immediately started fixing it.
I was told that the new sending units had the Gm 0 to 90 ohm resister even though they were marketed as mopar sending units. On a charger forum there were members there that swapped out the resister wiper and float with their old one. Thats what I did just last week because I had the same problem you did last year. My dash and gages were restored by a dash company so I knew my gages were ok. I haven't got my car up and running yet so I don't know if what I did will work.
If you do a search in the charger forum you might find the link.
I know you said that your old unit didn't read any differently but I just wanted to give you additional info for you to think about.

I woulod try moving the arm on the sender with it hooked up and grounded to see what the gauge does. That would tell you a bunch about what's happening.

I took the sending unit out and working from the front seat of the car, I tapped into the bulkhead behind the drivers side kick panel. When the arm is moved to full, the gauge moves to full. The gauge moves is relation to the sending unit arm until it hits 1/2 tank, then it drops down to empty. I went ahead and built the 5VDC solid state voltage regulator for the cluster, and tested the gas gauge with 2 AA batteries. With one battery, the gauge goes to 1/4, with 2, it goes to full. Since the gauges operate off of a 5V signal, and two AA batteries making 3V together, leads me to believe the gauge is bad. Does everyone agree with that? I checked the wire again from the 5-pin plug on the back of the cluster all the way back to the sending unit with a DVOM...the wire is fine. The sending unit and the cluster also have good grounds. So, I guess I have to try and find a gas gauge now, which should be tons of fun. Or does anybody know of anyone that rebuilds/repairs Mopar gauges?

My personal feeling is that if they move at all, they are good.... and you can adjust the fuel, oil press, and temp gauges.
they sense resistance to ground. there are set value resistors you can buy from radio shack very cheap. ground each gauge with a wire that is split by these resistors and then adjust the gauges to read properly.

Have you removed, cleaned and disassembled your gauge cluster? before giving my 68 charger gauges a makeover, I had some intermittent issues. I found that just by removing the cluster, taking apart, cleaning, and putting it all back together ended the problems. while they are out, you can make up your rig to test and adjust. very easy to do.
A voltage limiter needs to be plugged into the circuit board. Attach a 12v power source to the circuit board pin that supplies power to the board. Ground the cluster housing.

Go to Radio Shack and buy these resistors.
271-1101 10 ohm
271-1103 23 ohm
271-1109 150 ohm

Just attach a short piece of wire with a small alligator clip on each end to a single 10 ohm and 23 ohm resistor
Take two 150 ohm resistors and put them side by side and then attach the wires with clips.

The jumper wires are so cheap you could make three of each and have all your gauges testing at the same time.

The LOW position is 73 ohms
The MIDDLE position is 23 ohms
The HIGH position is 10 ohms

Temperature gauge
The L position should read 120* on the dial
The M position should read 170*~180* on the dial.
The H position should read 250* on the dial.

Oil gauge
The L position should read 0 psi on the dial.
The M position should read about 30 psi on the dial
The H position should read 80 psi on the dial

Fuel gauge
The L position should read empty on the dial
The M position should read just under 1/2 on the dial
The H position should read full on the dial

look at the attachment..on the back of the gauges are two little round "windows" inside each is a 'wheel' that you can turn to adjust the sweep of the gauge.

Excellent info and If that'd make the problem gas tank sending units read correctly that would sure solve alot of peoples issues with them.

was poking around my hard drive and found this great picture someone posted at another forum I go to. The previous info I posted about the radioshack part numbers and testing procedure is from there too. I adjusted 2 sets of gauges like this and it was very easy to do.

Here's how to do one adjustment to your temp gauge for example:

If you set up your cluster on the bench just like this one with volt reg plugged in, apply 12v to the 2nd pin in from the right. attach one end of a jumper wire to the pin in that same row that goes with the gauge you want to adjust, say pink for the temp. clip the other end of that jumper to a Radio shack 271-1103 23 ohm resistor. attach a 2nd jumper to the other end of this resistor. you will have one end of one jumper hanging in the breeze at this point, run that end to ground.

at this point, you can use the little adjusters on the gauge to make it read 170 to 180. then change the resistors out for the other two settings and adjust the gauge according to the chart.

I found that the ones I did were either very close or way way off. there didn't seem to be a middle ground.

Excellent procedure. here's the problem: with some of the aftermarket gas tank sending units haveing different ohms than oe will adj the gauge w the resistor then give a incorrect reading or would it be better just on these problem senders to mock it up w no resistor directly back to the tank then adj the gas gauge or will the resistor/adjustment give an accurate reading even w the sending unit ohms slightly off? I dont care so much about the F side accuracy but when it hits E I want about 5 miles of traveling left in the tank.