Welding aluminum with those products is really tricky.....

I make my mufflers either out of gutted stock mufflers, or out of steel. I really think the steel mufflers I make are about the same weight as the aluminum ones you buy because they are made of thinner stock. One of these days I'll do a weight comparison, but I don't feel like it now because the only "store bought" muffler I have is on my cowled, Ryobi powered, Tiger Moth. Don't feel like taking it all apart to weigh the muffler.

AV8TOR