I have a 75 Grandville with a 455. I am looking for an exhaust manifold. I understand the pontiac 455 block is the same as all the other large V8'* (350 and up). I assume that an exhaust manifold for any of these engines would fit. Does anyone know if the manifold from a Firebird of Lemans would also fit?

It'* not the block you need to be concerned about, it is the heads since the manifold bolts to the head and not the block. And most of the Pontiac V8s used the same exhaust manifold flange. What won't work is 301/265 or round port manifolds. Otherwise what you have heard is correct for the most part, that the Pontiac V8s all share a similar overall engine design. There are subtle differences between blocks of different displacements but most of them can interchange most of their parts.
As long as it is a D-port flange, it will bolt up to the engine fine but the outlet may be pointed wrong for the frame in your car. As long as the outlet comes out in the same place on both the old and new manifolds, you're golden.