a permanent metal to metal adhesive for 18 gauge sheet metal. For an automotive application, do not want to weld. Been considering 3M 5200FC as I know it works great of fiberglass and is rated for metal but I can't find much info on the metal use.
That Loc-Tite stuff is industrial strength, but they had something else for metal that looks pretty good, and it should be in hardware stores. Just need to check the max temp it can be exposed too. The 8700 strenght falls of rather quickly with higher temps.
J-B Weld is great stuff, but not sure it would hold 2 pieces of sheet together. I've used it as a filler.
46er, do you have an automotive paint supply house by you. 3m has a two part epoxy that is used for bonding body panels on automobiles. 3m Automix panel bonding adhesives is what you are looking for.