Try a gal of this stuff. Pour it full strength into the tank. First remove the tank from the car and plug the drain port. Make a metal cover for the fuel level sender and seal that also. Then cover the fill pipe with a piece of hose that is plugged with a piece of solid dowel and hose clamped in place.
Now put the tank into the back of a pickup truck, (tied down) and drive it around for a few days. The sloshing around will work the full inside of the tank. Now drain the B12 into a container and inspect it to see if the gas resin has been dissolved. If it worked a bit repeat with the same B12 and a longer time period. If it's really bad maybe 2 or 3 gals more of new B12.
This should work, I used it once to clean out a lawn mower gas tank and after a few days riding around in the back of my pickup it came out pretty clean and usable.
Once you are done, rinse out with a couple of gallons of gas. Do something with the contaminated gas that is good for the environment..............
Do I need to tell you to be careful with all that flammable fuel? Naw I guess you got that. Berryman B-12 Chemtool 0101 - Carburetor, Fuel System & Injector Cleaner | O'Reilly Auto Parts