Zombie Dart! '72 Dodge Dart

Good. Now lets hope the upper area's are solid. In my experience it's usually a seam near a tire that leaks first.

If it's good then you can use the eastwood sealer. Will be the cheapest alternative. Next step up from there is another gas tank,lol!! Gas tanks can be welded. I have done it. Best way is fill the tank with water or car exhaust. You may also be interested in a product called seal all if the holes are not big.

Also let me know if I can help with getting you a tank! Maybe somebody can donate a gently used one?