Gas tank repair?

[Another Jeep]

I need to repair a custom fuel filler neck where it adapts to the tank. Unfortunately this is a custom jobbie and it's a metal tank with a plastic neck. I need something that's flexible but still seals and will hold these two pieces together. Of course it also needs to be impervious to fuel. This isn't a flat surface repair or a simple plug. Any suggestions or advice on quality product? What kind of magic epoxy or putty have you used?

Thanks!