Feb 2006
-----------------------
the standard stainless
for many applications was 301.  This was indeed used for wheel covers,
as well as aircraft parts, architectural items, roof gutters, and in
particular railroad car bodies, because of its high strength
properties.  Type 301 can be cold worked to tensile strengths in the
region of 150,000-200,000 psi.  It is a 17-7 alloy (17% CR, 7% Ni), thus
slightly cheaper than some of the 300 series.  As someone noted, it
becomes magnetic when cold worked.

For automotive moulding and trim the usual alloy was 434.  This is one
of the magnetic 400 series which have no nickel, and 16-18% Cr.  Type
434 has about 1% Mo added for corrosion resistance.

FWIW, another of the 400 series, Type 409 is usually used for auto
exhaust.  It is a cheap stainless (thus its use by the auto industry)
with only 10-13% Cr (and no nickel of course), but serves the purpose I
suppose.  For serious exhaust work, such as aircraft exhaust, either
Types 321 or 347 are used (347 has somewhat better corrosion
resistance).
--
but generally
speaking 434 is a specialty product likely not easy to find.  In the 400
series 410 and 430 are much more common.  301 has the least corrosion
resistance of the 300's, but it can be cold worked (with some
difficulty) to high strengths.  It is weldable.  More common are types
302 and 304; they are weldable and have good 'deep draw' properties. 
Both can be considered 'all purpose' stainless.  (302 has a couple %
less Cr) and are easy to find.

Small sheets of the more common stainless varieties can be bought from
McMAster-Carr in many different thicknesses.  If you are going to be
using 1/8" sheets you will need either to cut and weld or have access to
rather large forming tools (note that stainless railroad passenger car
frames were formed from only 3/16" sheet).  

Many years ago on a research project we were quite interested in the
corrosion properties of materials as we were working with Hydrogen
flouride (HF) - very nasty stuff (the burns never really heal
properly).  By profession, before retiring a couple years ago, I was an
experimental research physicist (a couple of us serviced a lot of
Studes/Packards in grad school to pay bills).

-- wf.