have not used conduit, My dad stole my mom's chrome vacuum tubes off her tank vacuum years ago and used them for exhaust pipe on a project! LOL... He might have gotten in trouble for that one!
I would be concerned with fumes off the header pipe where it comes out of the manifold if using conduit. It may not be any big deal, but pulling a good size hill, it may get hot enough to stink! You may be able to get exhaust pipe lengths from your local Napa or other parts store.
