You should be able to solder tank mounts with a 40W iron if the tip isn't micro-sized. Let it get hot, then pre-tin both the mounting tab and the tank area. Then locate the parts. I use a stainless pan head screw thru the mounting slot into a piece of wood so I'm not fumbling around with loose parts. Then you can improvise something to press the tank up against it. (Thumb, rubber band, magnet, wood clamp, etc.) If I am making a tank to fit a screw pattern already on the plane, I just layout the same pattern on the soldering board, predrill some pilot holes, and jig everything up. Guarentees that it will fit and not be crooked when I am done. To get better heat transfer from the tip to the parts, heavily tin the tip of the iron. If everything is clean, it usually goes together no problems. One other thing to check is that the tip is free of scale and rust and firmly attached to the iron. A poor connection here will make even a big iron wimpy.
Epoxy is great stuff, but a good solder joint is probably easier to achieve than a good bond joint. I suspect it is also more tolerant of flex-induced failures--but I have no data on that. On the other hand, how many metal tanks can you buy that are glued together?
Dave "McSlow"