Hi.
I would exclude the option n. 1 because epoxy does not resist glow fuel.
Personally I use a 70 Watt soldering iron, but 40 should do the job.
I don't like welding gun because the tip has little mass, so it is difficult to heat the area to be welded correctly and on objects that dissipate a lot of heat I have never been able to get good welds.
On the contrary, with a Bernzomatic torch there is the risk of heating everything too much too quickly, with the result of melting the pond on too large an area and untiing part of the tank.
Massimo