coupla things...

After checking the grounds and cleaning up all the contact points and checking voltage from the Alt. and regulator for good contacts or swap.

Theres also a metal strap that conects the fuel line at the rear of the car on the fuel line. This simple strap bridges the rubber line that connects the gas tank to the fuel line that runs the length of the car.

The float could be stuck.

The Alt. gauge jumping around is showing that the system is being taxed. This is normal on the older cars. The Alt.'s were not very powerful. It does this on my Duster (74) and Cuda (73). The one in my Dodge is dead. Both have something like 60 amp Alt.
I have to be honest, I keep forgetting how strong the Alt.'s are.
In My Duster, the Alt. is from '79, as is the drivetrain. It has the 75 amp Alt. and it still moves around. (The gauge)