OK, hate to be the spoiler and I do hope it works for you but...
Plastic fuel tanks are made of cross-linked polyethylene. Once they are cast in the mold and as they cool, the molecular structure cross-links making it very strong. This is why these tanks don't respond all that well to heat guns or plastic welding. To get adhesion you would have to heat the surrounding area back to melting to unlink the chains and then apply the "patch" at the same temp. so that everything will link and bond completely. (and to be really honest, I'm not even sure this is even possible with this material).
On top of that, the used tank is likely saturated with gasoline & oil chemicals that have leached into the plastic, making it even harder to get anything to bond...
good luck!
BTW, I used to design gas tanks for boats :wink: