The original cork v/c gaskets that old are always dried up, shrunk and hard as a rock. Then you try to stretch them to make them fit and they just break and disintigrate. I think someone on here mentioned soaking them in something first to make them more pliable. But when you can go buy a set for a few bucks at the NAPA and shoot 'em with silver paint, why bother?
The other gaskets (metal head gaskets, compound intake gaskets, and rubber pan gaskets and valve seals) are usually still good though, and could be worth a good bit depending on the part # on the package and the year of manufacture.
Good luck!