[alien]

I looked into it - they need to be certified to weld pressure vessels, then the tank gets xrayed and certified by someone qualified to do so...

there's places that specialise in custom tanks, usually for trucks.

After all that, an engineer signs off on it and you go over the pits.

[Gwagensteve]

I'm not taking issue with the majority of what you're saying there Alien, but I can assure you an automotive fuel tank isn't subject to X-ray. The configuration of the welds doesn't lend itself to X-ray.

Magnetic particle inspection is more likely, which would be much cheaper.

I'd imagine 100% X-ray of something the size of an automotive fuel tank would cost in the region of $500-$700.
Magnetic particle inspection would cost in the region of $150.

I imagine for a one off tank to gain approval 100% MPI would be required, along with welding by a qualified welder welding to an approved procedure.

[alien]

You're probably right - i just remember it needed "testing".