With the heads installed on the engine, you can try dry fitting the intake with only the side gaskets under it and see how much mis-match there is on the intake bolt holes versus the threaded holes in the heads. Based on this and assuming there is a mis-match, the intake manifold sides can be cut so that the intake bolt holes are in alignment with the holes in the heads. The bottom of the manifold may also have to be cut depending upon which Pontiac engine is being discussed.