There's 2 bolts that hold it in. They can be hard to see from the top, as they sit just below the diaphram housing. Looks like you may have a dual action pump, which provids vacuum for the wipers. Not sure if that type is available new, but you should be able to get a new standard type pump from the local parts store (the may have to order it).
While a smallblock pump looks similar, only turned over, it is actually different. They do not interchange.
