If the interior of the tank is as bad as you describe I would not waster any more time and effort saving it. Unless the shop cuts the tank apart they will never be able to remove all the sealer that someone put into it. My guess is that there is some sealer still adhering to the walls but it will keep flaking off after you start driving the car.
I kept trying to “save” the original tank on my ‘37. Every 2 or 3 years I would have to remove it and have it cleaned. I also had a Fram G5 filter at the tank outlet that I had to change every 750 miles. The tank kept shedding a very fine rust.
If I had kept the car much longer I was going to replace the tank. I had way more money in it than the cost of the new tank.
| 
