I don't think aluminum headers would last long without melting, but I've seen a few people do the exhaust from the header back in aluminum without detrimental effect. The big problem is that Al looses its structural integrity at a low point, so while it may not melt, it will definitely crack if the whole system isn't fully supported. IMO aluminum exhausts sound pretty shitty though since the metal is so soft. It lowers and muffles the exhaust tone. I'd say Titanium is the way to go.