Failed sending unit signal is frustrating for sure. I too have a new wiring harness, new gas gauge, and new sending unit on my ‘57 3200. I tried the direct ground suggestion and my needle went to empty. Does that mean my wires are reversed on back of gauge?
At empty, the sending unit in your gas tank should read 0 ohms or as close to zero as possible...it may still show an ohm or two of resistance, but that's ok for now. A reading of zero ohms means no resistance, and that means your sending wire (which goes to the side of your gauge with the red tag) will be sent straight to ground. And this will make your gauge read empty. At full, your sending unit should read 30 ohms or as close to that as possible. That means your sending wire will be connected to ground through 30 ohms of resistance, and that will cause your gauge to read full. So it sounds to me your gauge is doing what it should and now the only thing you need to do is check your sender to see if (at full) it is giving you 30 ohms. Some of the sending units I've seen are made so poorly they don't even come close. I've suggested a very accurate sending unit about a dozen times here. The seller's ID on eBay is renbusan and the 0 to 30 ohm sender is made in Taiwan. For $29 it is excellent and should last a long time.
