When I was installing EFI, I installed a new tank and sender. I also put 2.5 gallons of gas from a can into the tank. The gauge read correctly; it just moved a tiny bit.

 

Aftermarket fuel sending units are a known problem. Apparently no company can manufacture them to match Ford specs. Your best bet is to locate an NOS unit. It will cost you some bucks ($300 + or -) but you won't have any headaches with it.

 

Aloha @ashtx The problem is not the sending unit, it's the gauge.

 

I've tried to adjust a gauge using a variable power supply and discrete resistors- while on the bench, and it is difficult to say the least. I managed to get it back to the factory setting after much fiddling around. If you are convinced its the gauge, I would send it to Rocketman or some other service for calibration.
However, I suspect its the sender. I have the advantage of being the original owner of my car, so I know what is factory and what is not. I just recently restored my original 69 sender and found that from the factory the float is about 1/2" off the bottom with a dry tank and with not quite a gallon (7 pints) the float begins to move up and starts to reduce the resistance. Here is the resistance vrs. gallons curve for a stock sender:

 

So, a 4 ohm increase in resistance.

Not good. The wire between the gauge and sender should have about a 0.1-0.2 Ohm resistance MAX. Also check resistance between the fuel tank and ground. What is the state of your engine-to-firewall ground strap? 58.3 Ohms at the sender should result in a gauge reading of around....

 

Aloha @Mach1 Driver. Thanx for the info. But, I do not think the issue is with the sender. When I tested this, the resistance at the sender was 58.3 ohms. According to @Woodchuck's fuel gauge resistance chart, that is somewhere between 1/4 and empty. It took 14 gallons to fill the 16 gallon tank, so there was ~ 2 gallons in the tank, which corresponds to ~ 1/8 tank. The needle should be indicating 1/8 tank, but it does not move from power off position. The sender is doing it's job, the gauge is not. Here's Woodchuck's chart:

Aloha @Woodchuck. Yes, that's what I'm hoping my gauge will indicate when down to 2 gallons!

I'll check the fuel tank to ground reading soon. The engine to firewall strap is new, installed last year, bought it from NPD. I put a star washer in between the eyelet and the firewall to ensure good contact. On my '65 I think there was a separate ground wire from the instrument panel to a support strut under the dash, but it's been about 30 years since I was under that dash so I could be mis-remembering. Nothing like that on my '68, and I don't see anyplace where one might have been. Is the '68 instrument panel only grounded through the wiring connectors?

Also, 4 ohms is really kind of strange to me. If the wire was damaged someplace between the sender and the gauge, I could easily believe a few k ohms. But 4 seems strange. I'll spray from DeOxit in the connector by the dash just in case it's corrosion (I live near the ocean so anything / everything corrodes). Other than running a new wire from the sender to the dash, any other suggestions?

Mahalo, Harry Z