I honestly couldn't find where the gauges in the dash are grounded at (if they even are?) and I assume the fuel sending unit is grounded to frame but I'll have to check tomorrow try testing what Tim suggested.
50ChevyGuy,
I do not know which gauges might have to be grounded by way of the gauge housing, but the lights would be grounded through the gauge housing (if the gauge lights go on, the housing is grounded).
Use the same diagnostic for this (as I recommended for the sender that resides in the fuel tank).
For the gauge housing: turn on the lights; then, run a grounded-wire from any clean-metal spot on the gauge housing to a body ground. Do this by hand - you will not get shocked if you hold the insulation on the wire.
A good spot to hold the wire onto the gauge housing would be against a bulb housing (that is where they make their ground). Run the other end to the ground terminal on the battery.
