I managed to gather up alot of plate that had the holes cut, and pipe welded in out of the scrap in our welding class.
Not sure what kind of steel it is, not too heavy, looks a little bit thicker than mild steel. Have enough to make two very narrow mufflers out of it.
I wouldn't worry about rust either, its going to take a long time for anything more than sheetmetal to rust through too horrible. Just paint it with some Wal-Mart header paint and it shouldn't get too bad.
That stuff actually sticks also, worked for about two years on a log truck with crappy, mis-matched mufflers with no prep work at all. Didn't burn off.