Thanks everyone
First, I think that I've found enough information to satisfy my immediate needs. So thiank you.
I had hoped to find an authoritavie resource or study that specifically address high temperature strength properties of sheetmetal type materials. Often, published high temperature data is for structural steels used in buildings are are specifically related to strength requirememts during fires. There is some data out there on specific high temperature use steels, or stainless but much of that info on the internet has been generalized, published by company marketing departments, or else not specific to steel types that are in everyday use for small engine exhaust systems.
To answer the question about types of steel, we use 304, 409, 430, stainless materials and also aluminized CRS materials of which base steel is probably of a commercial steel (?) type.