A 78 block will only have the three bolt mounts which won't directly bolt up to the firebird 2 bolt motor mounts. You will need some adapters that are available. I have no experience in using the adapters so somebody else will have to chime in.

Blocks up through 69 had two bolt mounts. 70-74 have five bolt holes, fit the old 2 bolt and newer 3 bolt mounts. 75 and up have just the 3 bolt holes. (or about those years)