An idea just popped into my head due to this thread. I've been wanting to up my downpipe size to 6" for some time now, but 6" mandrel bent stainless is hard to find. The idea of V-banding to an aluminum section is great! I'd bet finding mandrel bent 6" aluminum tubing might be easier to find.
diesel exhaust vendors or suppliers maybe? quick search found this
Grand Rock Chrome Exhaust Stack Tip
4"-7" sizes avaiable, I don't know have a quick fix for bending that pipe but cut n weld could fix most of that, but as you know it would be a lot of work.
6" diameter, 36" long, curved top for $170 looks like a decent bet