BigMo, when planning the layout for my stack exhaust, I had first figured on using flex for some of the bends. A friend, who is a big rig mechanic, told me the pipe has to be straight. Exhaust turbulance and cracks will occur if the pipe is bent. The Donaldson and Nelson exhaust catalogs both say the same thing.
I am not an exhaust expert, but just repeating what I have read and heard from exhaust parts manufacturers and a knowledgeable mechanic.