76 Formula Update and Sending unit question

[daleone3]

You guys have been great with helping me do some sorting out and troubleshooting so have another question.
On the 76 Pontiac Formula, have installed new fuel pump, tank and sending unit. Lines are clean and it is running magnitudes better, like a 8000 mile Pontiac should.
Once it was all together, I put in a can full (four gallons) of new fuel and primed the lines and started right up. BUT the fuel gauge is reading below empty. It sits below empty and when started it does not move at all.
I have power at the connection in the truck and the connections on both ends are clean.
The ground was reinstalled where it had been and the surface was clean.
I KNOW now that I should have tested the sending unit prior but I didn't so now trying to sort this out before I go to the knuckle cutting process of lowering the tank.
What is possible that the connection on the top of the tank sending unit may have gotten dislodged though I won't be able to check that until the endoscope arrives that I ordered, figured it was worth $60 to buy before I get too far. I really don't think I dislodged the connection and the ground seems to be fine so is there a way to check the actual fuel gauge without getting in to the dash? Any other suggestions?
My initial thought that being a 21+ gallon tank, the four gallons may not have been enough to raise the float but I don't want to put a ton of fuel in to it in case I need to drop it.....see my problem? I would think that four gallons would raise it enough to create some resistance output at the gauge but not certain.
Any insights in to the path of least resistance?
Thank you a ton.
Scott