JB Weld is iffy.  It might hold up it you keep the engine rich and cool.  I've also seen it fail in 30 seconds.

Conley Weld is better.

DevCon High Temp Epoxy is will work for sure.  Unfortuately, it doesn't seem to be on the market anymore. Marvin Denny was kind enough to give me what he had left.  I should have stocked up in '74.  That stuff never fails, regardless of temp.

Usingaluminum and brass together may be enough to give you grief. Those two metals will expand at very different rates.  Enough so that the header may self destruct regardless of the epoxy used.

To say nothing of the corrosion from using dissimilar metals. Ty's Navy experience shoulda reminded him of that. Even stainless steel and aluminum causes a "galvanic reaction".  Scott Dinger's mufflers are nice, as are Randy Smith's and "Big Art's". Pick one, and be happy!  Steve