Did you not mention that you have another junk sender?
Try this
1 Measure the sender resistance AT THE TANK between the metal of the sender itself and the sender terminal.
2 Measure the resistance AGAIN from the sender terminal to "stab" the other probe into the trunk floor. It should not change much / at all from step one
3 Measure the resistance a THIRD time from the disconnected gauge wire up at the gauge end, to the dash ground. Once again, it should be very close to step one
Now using the chart I originally posted, estimate what you think the "gauge should read" based on your sender test.
Before you hook things up, check the following:
(Don't know, is your cluster one or two PC boards??)
On my 67, I had "some" of the following problems, which you need to check
Loose, bad connections, broken pins at the PC board connector
On my board, the brass "springy fingers" that forms the contacts for the limiter were NOT making good contact with the board. Solder them TO the copper traces, OR use light ga. short jumpers to solder between the finger contacts and the PC board.
The gauge studs may not be making contact with the board. You might replace the nuts with new "Real" nuts, and use star washers. Work them loose/ tight a couple of times to "scrub the connection clean
Check carefully the PC board path to the connector pins, and use the "wiggle" test to look for bad pins. Be sure the board is WELL grounded to the casting, and use a separate ground wire from one of the board ground screws, bolt this wire to the column support behind the dash.
So hook things up, and allow as much as a minute for things to stabilize.
Now take your meter and read the voltage from the gauge sender terminal to ground, and make CAREFUL measurements to hundreds or at least tenths of volts Be absolutely SURE you have a good probe connections to the dash frame with the meter.
Now move to the dash PC connector and see if you can detect ANY voltage difference.
Move finally to the tank sender itself, and measure the voltage from the sender terminal (using a clip lead) to ground. Check BOTH the sender metal itself and the trunk floor.
Next, after letting gauges stabilize, measure the voltage at the temp and fuel limiter terminals. (It would be good to devise a way to get the temp gauge upscale, you can use your junker sender for this?
Now, after measuring the sender limiter voltage, DISCONNECT one or the other gauges, and see if the limiter voltage changes (I'm assuming by what you said earlier that you are using a solid state replacement limiter?)
