Gauge gremlin

[2manybowties]

I recentely purchased a 1971 chevy pu. its a short wheel base stepside 350 with a 3 speed on the column. When I turn my headlights on my fuel gauge goes to empty. I have checked the brown wire to the fuse panel, when I disconnect it the gauge goes to the 3 oclock position. I feel it is a ground issue. I am not sure if the truck has a ground that goes from the engine to the firewall would that cause my issue? Any help would be appreciated!