i am still digging but about a year ago there was a debate on one mod
list about the merits and demerits of stainless.  one of the members
posted a note that was from a stainless tubing mfr booklet to
determine the type of stainless you needed.  it varied by type of
flowing material (gas or liquid), temperature (very low to very high
or both), electrolysis effects, PH, pressure, and a couple other
variables.

to put it simple, exhaust gases would be extremely corrosive and
would eventually take out most standard stainless materials except
some of the pricier stuff.

engine coolant if kept with its proper level of anticorrosives and
proper rubber separations from dissimilar metals would last for years
even at some lower grades, but the electrolysis effects and
temperature variants suggested higher grades for that use as well.

the suggested alloy that i found in my old notes was 409

searched on stainless 409 and found

http://www.hghouston.com/ss_comp.html

very interesting table

same site as

http://www.hghouston.com/coppers/corrosion_cu.html

which is the table i used to determine that use of copper pipe for my
cooling lines was just fine back when i was doing my conversion.

just found some of my old notes

http://www.azom.com/Details.asp?ArticleID=969

some stainless do rust as part of their nature.

409 is a titanium stabilised ferritic stainless steel. Although
regarded as a general-purpose chromium stainless steel the primary
application for Grade 409 is automotive exhaust systems. Its
applications are those where appearance is a secondary consideration
to mechanical properties and corrosion resistance and where some
weldability is required.

More highly stabilised versions of 409 are useful where 409 has been
proven marginal - these grades S40910, S40920 and S40930 are
stabilised with titanium, niobium or both titanium and niobium

see the link for more details.

jimt