I've read a few of the books on building RMH and I have the DVDs
My main idea for the use of castiron in the build because of a few things:
The use of a long turn 90 would make it draft easier.
A 6" round has less friction loss than a 6" square does.
Cast iron is what wood stoves are made of.
I'm a plumbing and and HVAC tech. I live in a small town area so I get to wear lots of hats.
Also should have done more searching on my laptop as opposed to my phone. There are a few threads on this subject out there I failed to find. Thanks all.