CaptainBrad, This was a great post!!! It answered all my questions!! My exhaust pipe got so corroded that it came apart when I started the engine. I'm glad to see that I can use galvanized pipe and now I see that I do have to put the "pipe within a pipe."
I have a 1972 Ericson 32 and, fortunately, I have a copy of the user's manual which includes diagrams of the pipe with all the measurements. They call it a "Heat Riser."
I don't suppose you took any pictures of your exhaust pipe that you made? If so, would you be willing to share them? Is so, you can email them to email@example.com