-420 degrees celcius, is a little bit low. I think you've made a typographical error.

Sorry, I meant -420 F not C!

I would be surprised if any one has any test data for CTE of stainless steel that close to absolute zero.  I only know of 2 fluids go that cold and I assume since you're in aerospace, your fluid is LH2.  

Marshall Space Flight Center, being the propulsion directorate, might have information that could help you.  I know they have done extensive testing with LOX, but I'm not sure of information dealing with LH2, because of the explosive hazards associated.  They have a website.   The dealings I've had with the people in their metallics branch, they are very helpful.

If you have not found a sastisfactory value, you might check the data compiled by the cryogenics group at NIST.
http://cryogenics.nist.gov/
There is a link to materials properties data (Themal Conductivity,  Specific Heat &  Linear Expansion) at cryogenic temperatures for commonly used materials.  There is also a copy of a paper describing the database project and its use under publications.  

I use a materials database with temperature dependent properties, it not free but its fairly comprehensive.  try http://www.jahm.com