I know this is an old thread but what I have to share may be helpful for someone down the road. I have built many fuel tanks in my lifetime and I always line them with "3M Fuel Resistant Coating". A quart is usually more than enough to do most fuel tanks and it costs less than $100. In my 1952 Jeep, my original tank was lined in 1976 and is still in good condition as is the same for the additional tank I built for it mounted under the back. It is very easy to do yourself. After making certain the tank is as clean as it can be, cover openings and pour the material in and roll the tank around to get all the surfaces coated. Then pour the excess out and back into the container to save for additional coatings. They recommend two but I do three applications letting the material set properly between applications. Let the tank dry well before filling with fuel. This is probably the best product out there.
If you have a tank built out of stainless make certain is is made of 316 stainless. Anything less will eventually rust.
I am in the process of building a new tank for my 1926 Buick using mild steel which will be lined as mentioned above. If my mild steel Jeep tanks are still going strong after 45 years it says a lot for the 3M product.