Aloha,
I would recommend you follow Mickey and Randall's advice to help isolate the problem. A good path to ground often corrects electrical problems in LBCs. If the gauge's accuracy is the problem, the article you have and Frank referenced includes how to adjust it. It is not really necessary to disassemble the gauge or build the tester to make some adjustments to the readings of a working gauge. Remove the gauge from the dash and loosen the two nuts over the diagonal slots in the rear of the case. Do not remove them, just loosen a 1/4 to 1/2 turn so you can slide the coils back and forth. Using some extra wire connect the gauge to the car, you will need to add a ground wire from the case to a suitable ground in the car. If you know how much gas is in the tank you can slide the nuts (and thus the coils) as necessary to get a reasonable reading. As Randall said, this gauge is not always linear so if reads well on the upper half of the scale it may be a bit off on the lower half of the scale. Maybe the trip odometer is back up for an inaccurate fuel gauge.