With the signal wire grounded, the fuel gauge should indicate Empty. With the signal wire disconnected, the fuel gauge should indicate well past Full with the needle pointing ~3:00 o'clock. Correct the connection issues between the gauge pins and PCB. The clips (image) may need to be gently retensioned using needle-nose pliers. Don't overbend them or you'll introduce gauge fitment problems!
(http://forum.73-87chevytrucks.com/smforum/index.php?action=dlattach;topic=37117.0;attach=51117;image)
Excerpt from the Application Notes in "Functional Tests of Factory GM Electric Gauges" previously linked:
"The instrument cluster gauges (excluding speedometers, clocks and tachometers) establish electrical connection with the flexible printed circuit board (PCB) through shiny metal spring clips (Fig. 8) that snap through the back of the instrument cluster housing. The PCB and clips are notorious for accumulating dust and tarnish to the point of degrading electrical connections, while the spring clips tend to lose tension over time, degrading connections even further. Hence, anytime the cluster is disassembled for gauge service or new gauges are installed, the clips should be burnished along with the PCB where the clips make contact. But, be careful - the copper foil of the PCB can peel easily from its plastic backing. A soft pencil eraser or crocus cloth works well for burnishing the copper foil. Once the spring clips have been removed from the housing, they can be gently reformed to increase contact pressure with both the PCB and gauge pins. Furthermore, inspect and burnish the connections between the wiring harness plug and PCB."