Most GM part numbers from the SNL are not going to get you a FNS/NSN/NIIN number. One web site I found can take an old GM part number and tell you what the new NSN number is.
Go here:
http://www.google.com/custom?domains=ww ... 1%3B&hl=en
and play. Its been pretty accurate with the 3-4 parts I looked up.