I'm trying to find one and I see a few cheap firebirds for sale that I may buy and part out if it has the WC tranny. Any ideas on what years to get or to avoid?

 

W.C. came on 1988 and newer. That doesn't mean someone didn't change out a W.C. for a non-W.C. Probably the quickest way to tell is to pull a drain plug. If atf comes out you probably have a W.C., if gear oil comes out, it's not W.C. If someone is determined to rip you off they can always put atf in a nonW.C.
There are physical differences, but I'm not familier enough with them to try to describe the differences in the cases on the outside.