Most OEM installations were cast iron for durability over the long haul.
The most tubing header utilization from OEM I've seen lately were the 4.0 Jeep of later years where the material was 400 grade heat resistant "stainless".The things break all to pieces and are Hail to weld reliably.The manufacturer seemed to change designs annually trying to make them reliable and they even used CI at one time.
Tubing headers made of common exhaust tubing steel are short lived in hard work situations because of rusting away.All these materials run at red heat when working hard and the steel just rusts away because of the heating/cooling cycles.
CI is more heat stable and enduring but some manifolds/headers are GOING to break because of poor design.
The inline 6 or 8 is difficult because of the greater length making greater overall growth.
The clamped up manifold assembly like Chevy 6 is to allow slight movement to prevent breaking.
Then the good old "slant 6" Mopar tried to bolt it tight and had to do some unconventional bolting to keep it in 1 piece.If you bolt one of these hard and fast it will soon be a 2 piece manifold.
