Come to think of it, there is a DOT certification for fuel tanks because people converting to propane or CNG need to buy the more expensive DOT approved tanks rather than running plain old BBQ tanks.

I would seriously reconsider building my own fuel tank. Even if it is stainless, it will crumple in an accident. If it does, the best tig welder around can't keep it from cracking. If it cracks, gas leaks.

I would also hate to be in an accident where it catches fire and somebody else gets hurt. Lawyers would have a field day with the do-it-yourself gas tank you had.