Help with Temp, Gas, Oil Alt gauges FREE VIDEO INCLUDED!

The tank may be chassis grounded, rusty or not. The issue isn't the tank but the sender. A chassis ground path from sender to tank is questionable due to a seal/gasket between there. Then the rubber fuel hose also isolates sender.
Since the strap supporting the tank has or should have a rubber isolator between it and the tank, along with a insulating pad atop the tank, and a rubber seal at the filler tube, chassis ground at the tank is questionable. So that's why there isn't a male spade terminal made on the tank and a length of common wire from sender to the tank. A simple clip on ground jumper was installed from sender fuel tube to vehicles fuel line. Those other options were just too "iffy" .
There may be a OEM type ground clip available. Some owners have homemade a ground jumper using a length of wire, attached to the senders fuel tube with additional gear screw clamp, and at other end a ring terminal under a sheet metal screw placed in the vehicles chassis under there somewhere.
Right this minute your sender is grounded to the tank at that clamp ring and the tank is chassis grounded. This is shown/proven in gauge operation. Is there a 2 or 3 ohm resistance present that is effecting gauge accuracy? I don't know. Your ohms meter between sender face plate and vehicle chassis answers that question.