On a less harsh note, an explanation of why you cannot use the 2.5 gaskets. The bore diameter of the 2.5 is smaller than the 3.0. Using a gasket ment for the smaller diamer cylinder will result in some of the gasket hanging into the combustion chamber. Some of the gasket sealing design is probably in that area where it will not result in the appropriate "crush" to seal properly. More importantly, the gasket material hanging into the combustion chamber will very quickly burn up.
As soon as you try to lay the 2.5 gasket onto the deck of a 3.0 block you would see the problem.