There is a bolt going through the fire wall directly in front of where the inst. panel is mounted, you have to pull the panel to get to it, there are ground lugs on both sides of the firewall.
Put your meter in ohms (the sign of the omega) touch the probes together and note the readout, many times it will not be zero, then put one lead on the ground shunt Mounted on the inside towards the front of the battery box, then pierce the insulation on wire #58 at the sending unit and the reading should be close to what you read with the probes touching each other.
You can also pull wire #28 off of the sending unit and ground the end going to the gauge, open will be ether full or empty and shorted to ground will be the other, never can remember which as many types of vehicles are different, but you should see a difference between open and grounded.
You can also pull the wire #58 connection and ohm out the side going into the harness but I prefer to test all circuits in place, thus the piercing of the insulation