Tried to send this earlier, but it failed to 'go'.
If you can do the fabrication/fitting work, any halfway decent local welder with an AC TIG rig should be able to weld it for you. I doubt many would care, but just tell 'em it's an off road fuel tank. You can bet he's already built some for local racers, off-roaders, etc. Do tell him it's a fuel tank, though, so he'll test for pinholes in the welds.
+1 for Ed at Tech Welding, if you want to totally farm it out. He does a lot of work for alt engine guys, so he isn't afraid of aviation.
Charlie