I put a double size gas tank in my 75 Ford truck a while back. There is no sender that works for it and they send a chinsey "kit" to modify the old one. I didn't like it. I used a Bronco sending unit. Their tanks are 6 gallons less than the 39 gallon I have and I knew it would be close. When it reads "E" it's empty. When it's half way or more full, the gauge is pegged. That's fine with me. All I care about is when it gets close to E that it's accurate and it is. At least it won't leave me walkin. There's really "not much" to do except "get it close" and live with it.
You "could" send the old one out to get it rebuilt. There are several companies that do it. Some of them even want the gauge with it so they can calibrate the two together. Course "that costs".