Jdub, now I understand. Thanks for the thorough description. I got my bungs in order now.
The plan is starting to come together. I will locate the expander as close as possible to the 90 degree elbow from the turbine. The elbows I have seen have either a 4" or 6" leg so that should give me plenty of fall to clear the firewall. At this location, I will need to use a shorter (greater angle) expander because there is very little room for lengthy tangents.
What is the verdict on a bellows. Should I include and where. A 3" in to 3-1/2" out bellows would be useful for this application.