I used 1 3/4" galvanized fence posts to run my dual exhaust back in April. They had sat for almost 4 years behind a shed in my backyard and hadn't rusted so I figured, what the heck. You've gotta use a respirator if you weld galvanized metal though as it's toxic. If you can use clamps and no welding, even easier (although leaks will be prone). Auto parts stores will usually have small pre-bent pieces of aluminized if you need to make turns here and there.
-David
Kustoms-and-Sleds

Tradin' up dimes for nickels...
'`'`'`'`'`'`
My \'65 Chevy C-20