The pump flange could have a universal multi hole pattern. If you rotate the pulley, you may see a fit. Your motor is weird. Some parts are SBC, some BBC and some either/or. A bastard motor. The water pump seems to be for a BBC. 427, 454.
If the pilot diameter is not snug, Jerry's suggestion is a good idea. Whatever you use as a centering bushing can be removed once the bolts are tight.
If it were me, I would want the correct pump and pulley however.
