If you have access to a dairy supply place, or a larger steel supplier, you could go stainless tube. I had short runs of aluminum on the plane, after a number of years, it did show some corrosion in small spots on the inside of the tube. Even with the antifreeze 50/50 mix. A bonus is, the exhaust shops can bend the stainless with no problems, less joints, less leaks. As a side note. Had one guy make the headers for a Vee Dub powered plane from stainless. Had them tig welded, looked nice when done. Just thinking the piping could fit up nice and tight to the vehicle with the proper bends, would look like they grew there! Sniper