Dead on with this answer.
Just one very serious warning on this topic. You are proposing to weld a fuel tank, I have no problem witht that as long as you make sure that there are no traces of fuel or fumes.
The way I have always done fuel tank repairs, (on everything from chainsaws to race cars) is to steam clean them so that the tank itself gets hot and then weld them while warm. The reason for this is to drive out the vapours that will be produced when you start welding BEFORE you start welding.
True story - a co worker ran out of diesel for the steam cleaner so only pressure washed a boat fuel tank with cold water, he then drained it and proceeded to tack weld the two new fittings that he had to fit. On the fourth tack he performed there must have been sufficient vapour in the tank, which ignited and he was blown off his feet by the blast and the tank, constructed from folded 3mm aluminium was transformed from a cube shape to an almost perfectly flat sheet in about 1/100th of a second.
Don't let this be you, or your Jonsereds tank!
You may not have access to a steam cleaner but for a small tank like this, boiling hot water with a bit of detergent poured in the tank would probably suffice. If when you drain the water from the tank tank you can still smell fumes do it again until you can no longer smell fumes.
Drain it, clean it with hot water, weld it while still warm, leak test it - sit back and enjoy a beer.
