Hay again most dealer parts don't cross reference to aftermarket unless there was a program added. They are very proprietary. now from year to year the will usually list the applications they fit by part number, Ford will in some parts, although they many not cross between brands ie chevy to pontiac.
there is an on line GM alternative parts catalog.
http://www.gmpartsdirect.com/catalog...TOKEN=81083575
I have not researched this very well give it a try.
May be it's what you need.
NAPA had one of the best cross reference catalogs.
But the way vendors are changing hands about every 9 months it's hard to keep up.
I think TRW has changed hands 5 time in the last 15 years, including mergers and departments being sold and added.
One of the best actions is find a partsman willing to get in to that section of their catalogs and programs.
another thing I have done on line is register with a large chain, autozone online, look up one year of car truck, like a 1977gmc C15 then change vehicles to a 87 chevy C10.
in some cases it will show it will not fit like a alternator. it will if you squeeze down a bracket, or change a bracket but that's another story. I could go on and on but I think you get the idea.
Most stores cross reference for two reasons Grade checking, comparing quality by vendor. and price checking, some times at AZ you were told to check a price and avalibility ,to check against a price checking customer. and just in case the delivery was late to the store for a shop customer.