Here's another way to make up weird-shaped exhaust pipes:  Borla, a company that makes racing exhausts for cars, has a variety of straight and bent pieces in the same alloys, diameters, and wall thicknesses used in aircraft exhausts.  (They're on-line -- just Google 'em.)  You can cut these up using a chop saw or band saw, grind the ends flat (a 12" disc sander works great), then piece them together to make whatever snake-y sort of system you need.  Hold them together with tape until you're satisfied, then drill #56 or #60 holes in pairs across matching pieces and safety wire them together to hold the registration.   Find a good welder than has experience with thin-walled stainless to put them together for you.  For a few hundred bucks worth of materials and labor you've got what you need.
   It doesn't work to get straight pieces and try to bend them.  Thin-walled alloys like these need a "mandrel bender," which apparently costs many K$.  Dunno exactly how it works, but it somehow supports the material inside so that the tubing doesn't crimp or collapse when bent.  The benders you find in automotive shops will take your nice A/C grade materials and prepare them for use as yard art.

Jim Cameron
Boerne, TX
Legacy N132X