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There are many different specs from different sources.
The official GM specs are very broad, you can be within the GM specs and have either a very good alignment or horrible one.
The most popular specs seem to be the Pfadt Street specs, I use them and like 'em. They are within the GM range, so a dealer should be willing to use those numbers.
Just remember that you want a little toe-IN front and rear, never toe-OUT. The Pfadt sheet can be a little confusing on that aspect.