Yes, it is JB weld (the marine version) and it is not strong enough on its own for strength, I mainly use it to seal the structure. The front end of the tube is split and the header screws will engage i the Al-tubing too. Don't know what will happen in a crash but it kind of works for these small engines (here a Picco Zero 0.8cc car engine converted to airplane use).

The design of internals of the muffler all depends on what one is after, high power or high sound damping. For a boost in power, the inner tube should be long in order to emulate a folded tuned pipe. If one just wants a quiet muffler the header tube can end right inside the can. The position of the exit tube will affect the reflection in the folded pipe mode and the sound damping in a muffler mode. For more damping of the sound (in a regular muffler mode) the stinger can have its inner end starting near the muffler wall (where sounds waves have a lower pressure variation). It needs to start in the lower end though in order to drain out the oil.