Kevin, It may be the "Extension tubes" that live in the manifold and exhaust stream are a separate part that are Stainless & not the Trees>?
My 1968 396 Trees rotted from the inside out, definitely ordinary steel, with steel tubes crimped and brazed at the joints. I just gave up attempting to repair and got replacements back in the 90's from Bill H.
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69 Los Z11
68 SS/RS 396/325
68 Los Z28
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